I've found the Spas-12 to be
excellent in closer-quarters situations. Just about to get hardcore team deathmatch unlocked though, so it won't be as necessary anymore. I just hate regular mode...It shouldn't take me half a magazine from an M4 to drop someone. And some of my .50 Barrett rounds have left people alive (and not in last stand either).
Which is another thing, I think the gun selection in this game is rather strange. Of course I know the M4/16, AK, MP5 and so forth. But then all of these weird looking other guns. Vector, F2000, TAR-xx. Strange, and none of them seem much different than the next, unlike in CoD4 where I felt every gun had its particular purpose and sort of personality. So then they drop all but the most hardcore of the sniper rifles (seriously, you start with a .50 Barrett as an option? Besides capacity and less recoil, what more can you possibly need? In my experience, snipers aren't going to like this game as much. Most of the maps are very urban/crowded, or just don't have a good long line of sight where anything more than an assault rifle with ACOG can't do just fine (besides the power, see hardcore mode).
But I am happy that the shotguns are useful. I find that in too many games they have about a 5ft range of being deadly.
As for killstreaks, I'm not sure if everybody understood that ever single kill you make doesn't count towards it. The kills have to be done by you. Not by your killstreaks. So you kill three people, get your UAV, then shoot/stab/blow up one more to get your care package. The care package really is just any random killstreak reward (no nuke though

). Say you get an AC-130 from that, and you manage to get 22 kills from the AC-130. You won't get a single reward from any of those.
And from the start you'll only be able to have killstreak rewards at 3, 4, and 5 kills.