Yeah, yeah, it has problems, but dang it, it's FUN...
Yes, there are some bugs. Very few of them are gamebreaking, however...most of them are just more or less in the "amusing" category. For example, in multiplayer, it is not unusual for your co-op partner to appear to be holding one weapon, but is, in fact, holding another one. This is especially amusing when the phantom weapon is a flare gun or airstrike laser. Every now and again, when hijacking a vehicle, you can get stuck if the tank or chopper is destroyed just as you sit down in the drivers seat, and the only recourse is to exit the game and come back in...however, since your partner can usually stay in the game, it only costs you a minute or two. Annoying, yes, but shouldn't get in the way of the fun.
The other complaints...limited ammo? Yeah, of course. Should we have infinite RPG's? Airstrikes? They can be quite useful and there are a lot of creative ways to use them...fly a chopper on top of a targetted building, jump out, pop smoke, and fly away as an artillery barrage hits...good stuff.
If you liked the original, I think you'll like this one, as well. It has a few limitations, but I think the new content and mechanics go a long way in making it just as enjoyable as the first Mercenaries. It is hardly perfect, but as with many other "flawed" games, the important thing is that it's fun, and Mercenaries 2 passes that test with flying colors.