Earlier today I played some Assault on Salamun Market and Pyrrhus Rise on the Helghast side and looking though all the classes and I had to stop at the Assault class and say "That's a kick*bleep* overcoat" and clicked to play on that class. Luckily I had a game where all weapons are allowed and I got to try every weapon. I've originally heard that the Assault was unbalanced and generally unfair and I have to say that on paper, the class looks like it's a pain in the *bleep* to fight but I felt it much more balanced. The rocket launcher is slow, can be shot down, takes a while to load each shot, and mainly can only hold 3 shots at a time so if no one is repairing on your side you're pretty much screwed because of the lack of people who want to use the Assault class because of the lack of conventional weaponry and/or don't want to be whined at for, to quote those mad at me, having no skill at aiming.
The rockets can be used very well to solve one of the game's biggest problems: spamming bottleneck areas. 7 people in one room with shotguns? *fires* Solved. It makes the enemies either whine or make their movements a little more planned and not "Lets all run up in here!".
While the rockets themselves can score kills to a decent effect, without support the Assault class can get screwed over quickly. With the low ammo and length between shots and reloading with the grenade launcher the class needs that body armour, though once it's gone it's never regenerating fully for you again, it makes the body armour something you want to ration the damage of. People have also said it's too hard to take them down... they need it, I think. Group of people run at you, you know you'd be aiming at the guy with the rocket launcher first.
As for the Grenade Launcher, it does make me laugh that no one thinks about what it can do. For those who want to run-n-gun with the assault it is a great weapon to use if you can take enough time to reload and stock up on ammo between. You do slightly less damage than the rocket and can fire more at a time so I do laugh when I see the rockets all locked and the grenade launcher my only choice because that just calls someone to run-n-gun. Though, if you want to use it properly, use it to rid the game of turrets and take pot shots into crowds. You can shoot around corners and even though you fired the grenades on the ground the enemies will think you've missed and walk right into it.
Now, finally, for Boost. Boost is by far one of the most useful secondary badges and can be used in two different ways. One fierce but easy to screw up, and one safe and efficient. The former of the two is running out into a battle to flank or surprise the enemy. The last thing they'd think is someone is an enemy is suddenly up close with them and definitely early on in the game. This idea can be messed up terribly by suicidal Assaults who want to run through the front lines and do as much damage as possible and is the hallmark reason the class is so bashed (that and the rocket launcher, a weapon that has been labeled unfair since day one). The other option to use Boost is actually to help something I've been mentioning quite a bit: Ammo. I've seen Assaults use their pistol more than any other class just because their out of explosives so use your boost as an escape tool. Get on the front lines and hurt their defense and when it becomes too tense and/or you have no ammo use the boost and get the heck out.
In general I believe the Assault class is a fair class that many don't know how to use correctly and while it's generally easy to get a kill, it's harder to get less deaths. Surviving as an Assault class is a skill few have.
I main Medic and Assault now, both with the repair secondary.