So, I've played going on 5 hours of the Evolve Big Alpha this weekend so far, and my impression so far is that the game is on the right track to be a ton of fun. I've spent a fair amount of time with each class so far, and while it's not really possible to pick a standout among the hunters, I can say without a shadow of a doubt that playing as the monster is an experience far detached from all the other classes, and that's a positive thing.
The Big Alpha does allow you to play as any class or monster currently known to the public, which allows for 2 characters for each class as well as Goliath and Kraken for the monsters. However, each secondary character for every class (including the beast) is locked behind class challenges, and I've yet gotten any of them due to switching around a lot. However, I can offer my thoughts on each class individually in their default forms.
All Hunters have a couple basic techniques that they can all use equally. They both consist of the jetpack that each of them carries. Aside from offering simple flotation when you desire it, you can also use a jetpack to 'climb' any wall that gets in your way, even if you lack the fuel for it (though you'll climb slower in that case). The other great use for the jetpack is the ability to 'dodge' in any direction quickly, which can also be used as a dash when chasing the monster, though it uses a lot of fuel that is fairly slow to refuel.
Medic - Val
The medic class is actually a far-cry from what you find in most other class-based shooters. She has her healing abilities, of course, which consist of a Team Fortress 2 style medi-gun and an area-of-effect healing burst with a fairly hefty cooldown that heals you as well as allies. However, she serves dual-purpose as another sort of support class in the game as well. She is also equipped with a tranquilizer gun which slows down the monster significantly in its on-foot movement as well as giving it a green glow temporarily which is visible to your team through walls, effectively marking it. She also has a sniper rifle in her possession that does decent damage to the monster's armor, but less to his health bar. However, any sniper shot that lands leaves a temporary 'mark' on that point of the monster's body which lets any attack that lands there deal double damage. She's quite versatile, and if your team does a good job of avoiding damage, she can be a huge asset toward dealing huge damage. She is certainly my favorite hunter, personally.
Support - Hank
The support class is more than what you'd expect as far as his damage dealing potential. He has a laser auto-rifle that shreds through wildlife creatures with ease as well as does decent damage to the monster's health, though not so much to his armor. He also possesses an orbital strike to call in from above which rains mortars in a fair spread around the target, dealing heavy damage to any creatures it hits, and merely knocking around allies unharmed. His 'support' role comes in with his other two pieces of kit, however: the cloaking device and the shield. When he activates the cloak, it makes him invisible as well as nearby allies, which prevents the characters from being detected by a monster's 'senses', though their silhouettes can be seen if the monster player looks carefully. The shield, on the other hand, only affects one player. Lock the shield's beam onto a player to absorb any and all damage they take until the shield's energy runs out, or until your line-of-sight is broken by the monster's knockback. This is a pleasantly fun class in my opinion, but I don't prefer him out of the options available.
Trapper - Maggie
The trapper is the most unique character of the hunters, no doubt. she only has three pieces of gear as opposed to all of the other classes having four. Her fourth gear slot is taken instead by her pet trapjaw, the team's tracker named Daisy. Daisy will sniff out the monster's tracks if it is not sneaking, or the monster itself if it's sneaking nearby, as well as revive any downed players when possible. As far as the trapper's equipment goes, her most standard piece is her machine-gun, which does substantial damage to the monster's health but next to nothing to its armor. She also has at her disposal an 'arena' she can throw down, which will create an inescapable dome roughly 100 meters in diameter to trap the monster inside if the team wishes to fight it when it tries to flee. When in combat, she can be most helpful by launching harpoon traps on the ground which will latch onto the monster and keep it from moving far until it focuses at least one strike per harpoon to break free. This is also helpful when the team is trying to escape a battle. The trapper is my second favorite class to play when I'm a hunter because she is quite unique, but I prefer Val for her more direct forms of support.
Assault - Markov
Markov seems to me to be the most uninspired character of the group, and the least fun to play, though I know my opinion will differ greatly from many other players. His primary weapon is a 'volt rifle' of sorts, which shoots pure electricity that can arc between multiple creatures for splash damage. He also possesses a personal shield for use when he is in trouble which will temporarily nullify all damage he receives, which is of course very helpful. He has an Assault Rifle which is weaker than his lighting gun, but has significantly farther reach if he wishes to hang back when low on health but continue to deal damage. Arguably his best asset, however, is the ability to lay 'Arc Mines' which are simple explosive proximity mines which, if used intelligently, can deal huge amounts of damage to the monster's health or armor. I don't enjoy playing as him simply because he is fairly formulaic, but he's an excellent form of offense for any player who wants to simply dish out damage.
Monster - Goliath
The monster's job is significantly different than the hunters. Whereas their main objective is simply to find and kill the monster, it is best to initially avoid the hunters when playing as the beast. Your primary objective in the end is to either kill all hunters before they respawn (which will be a little over 2 minutes after the first one is killed, no matter how many die afterwards), or to evolve to 'Stage 3' and destroy the humans' Power Relay. You evolve into a stronger monster by killing and eating the wildlife around the map, and you get 3 'skill points' to put towards your four skills with each Stage, including the inital one, with a maximum of 3 points for each skill.
In order to successfully hunt the local wildlife without the hunters ruining your fun you have to focus on sneaking around, as any movement whatsoever while not sneaking leaves tracks that Daisy will find and will be revealed to the hunters visually. Any loud or sudden actions around flocks of birds cause them to fly into the air and alert the hunters, as well, and there are even 'carrion birds' that flock to kills occasionally if you're being so sneaky that the humans otherwise wouldn't have a chance. It's never unfair, however, and in my experience it's always possible to avoid the humans if you use your 'sense' ability that reveals all nearby living things intelligently and hide among the environment.
As well as getting energy for evolving through eating wildlife, you also increase your armor by doing so. Your health is unable to be replenished, so it is best to maintain armor when initiating combat with the hunters, though you start with no armor after every evolution, meaning you have to hunt more before a fight is plausible.
Overall, the monster's unique play style, as well as its versatility of ability leveling, creates an experience wholly different from that of any of the hunters, and it genuinely creates a new fun experience to switch to that is, in my opinion, the more fun to master when playing solo than it is to master a single 'class' with random allies who may otherwise drag you down.
The Alpha itself has actually had very few errors aside from minor HUD glitches, such as the mouse and keyboard controls showing up when I'm using a 360 controller, or the lobby HUD remaining on-screen for an indeterminate amount of time into gameplay. Otherwise, aside from an odd matchmaking issue that plagued the PC version on its first day which has since been fixed, it runs great. It is also a phenomenal time, and I would very much recommend looking into trying the open beta that will be releasing in the coming months if you're even remotely interested in it.