Pretty good, despite the bugs.
A bit easy, especially after the first, with the exception of the euh... ranged units you meet later on (to not spoil anything too much). Which I still don't realy understand, by the way. Overall except for those guys having Lee and Jameson at the same time was just too much firepower, especially as the aide can just die and respawn over and over with no penalty. Not sure how frustrating it would have been if he would have just died though. Maybe his firepower could get downgraded if he dies?
Oh, and I barely tried the swarm missions, just the first, where I died. They were a lot harder than the main game, but I didn't enjoy them so much. I'm sure some people loved them, and I'm sure unlocking all guns is nice, but really the AR, SMG and pulse rifles are all good guns which together give a good amount of ammo, big mags, and properly upgraded reloading is fast.
I like how the SMG was a lot more useful than in the previous game. However I'm not sure the pulse rifle that came later is the better gun. Liked the sentry gun. Not sure it uses any ammo, though, which is weird but ok. I don't know how I'd have have to recharge it's ammo anyways. Didn't get around to using explosives, though.
The new baddies and the new ambush styles were nice (water, etc.). The outside area with the shadow stuff was nice. I wish we had seen more new baddies, to be honest. Some seemed to come late.
As for bugs, I had an issue where the smg ammo would be set to 31 bullets left, if I was using another gun. This seems to happen wether it's full or empty, which can be annoying in some cases, as frankly, it was my favourite gun alongside the pulse rifle. Also I might be repeating myself but there is ammo listed for the sentry gun, but it just keeps firing. And when I pick it back up it is full. Might be better this way though.