This will evolve, as all class-oriented stuff does, as time goes on. Right now I have only had experience in two, Vanguard and Infiltrator. But I will make my best stabs at the others.
They are listed in ORDER of how I feel they should be viewed.
Also note that all classes, even your squad, dip into other classes and abilities more than you may think.
Infiltrator
Spec: Sniper Rifles and Cloak
Clearly the most over-powered. You have a vanish ability that instantly drops you from enemy fire. As well as allow you to enter enemy waters for first blood. And not to mention you use a sniper rifle, and your shots from stealth can be powered up. That’s sick and it allows you to be very reckless since you have a safety net.
And you have a sniper rifle slowdown effect when you first aim. Priceless.
You can also grab Incinerate and some ammo boosts. With incinerate you are a beast.
Just don’t forget to pop your cloak when entering uncharted areas.
Soldier
Spec: Guns and Slo-mo
You live off the slow-motion, as I understand, and you are just a shooter. Probably an extra biotic, but doubtful.
The assault rifles are balanced weapons, and if you can mix in the shotgun and melee with your slow-mo you can do sick things.
Sentinel
Spec: Biotics and Tech
I hear they are very capable of doing a lot against all enemy types and barriers. They must be weak with guns, but that is okay. The utility here is key.
Engineer
Spec: Tech
Tech is a very nice spec I found in my second play by using Legion and Tali a bit more.
Drones are excellent decoys, and combined with hacking you can tear up any groups of mechs.
Not sure how their higher-up stuff looks.
Adept
Spec: Biotics
I hear they are pretty limited in abilities and lack utility for enemy types.
I’m sure you tear up fleshies, but the hardest enemies don’t fight naked.
Vanguard
Spec: Shotguns and Charge
To be honest, my first run was this, and it wasn’t very fun for my only imported ME1 toon.
The idea is you use shotguns and Charge to close distance. Well, this game isn’t very forgiving for running into the open, so it’s kinda only a viable tactic on single enemies.
The shotgun did help against the waves of husks, and charge did help on some of the bigger goons. But this is probably the worst class of them all.
Shockwave and the biotics were helpful at times as well.
As soon as I got a sniper rifle I was happy. I guess this class would be awesome in some kind of player versus player.
New Ability, New Gun
About mid-way through the game you can choose a new gun type to carry and after getting loyal teammates you can choose one of their specials.
Warp ammo and Reave are nice choices.
Sniper rifle and an Assault Rifle are also great.
Snipers can consider a Window rifle with extreme damage.
But no matter what you choose for whatever your spec, just know that you are not pinned down to your class and you can mix things up.
And the biggest tip of all: USE YOUR TEAM.
