I became easily frustrated playing this game first because I was so used to the crap that was MW2. However, this game has grown on me quickly and I easily lose track of time while playing (was up till 2:30am last night).
I love playing TDM, but my favorite has easily become Demolition...just for the objective portion of it. Demolition can only be good if all the players are actually trying for the objective though.
I also enjoy going to hardcore matches sometimes. Make things a little more difficult with no auto aim or HUD. Unfortunately, hardcore can lead to tons and tons of camping....so it gets boring very quickly. Really just a change of pace.
I am barely through single player because I like to start on Veteran. It gives way more of a challenge and doesn't waste the entire single player in one day or week.
I get bored of zombies easily, but it is a fun feature. To be honest, I enjoy Dead Ops more (try it if you haven't) because it gives me that old school arcade feeling.
Top 2 guns for me...
Galil with red dot sight (green cross with dot in middle) and a suppressor.
Commando with a red dot sight is just a great, powerful gun.
...the Stoner is another good gun but the 2 mentioned are top.