Just recently picked this game up after originally deciding to pass on it and after almost hitting 2nd prestige (haven't touched any other mode yet), I'm very torn on it. The pick 10 system is super fun and perhaps the best new feature in any CoD ever, even though it has a slight balancing flaw in that it's kinda useless in levels 1-40 and you end up having classes with 8/10 because there's nothing else to put on, but the kind of variety it brings is still amazing. I also feel like this is the CoD that gets the closest to having actual weapon balance. Besides the Skorpion, which is far and away my favorite gun so far, I don't feel like there are any other obvious "best guns", like the MP40, UMP45 or FAD. I can kick some ass with MTAR, Type25, AN94, MSMC, MP7, S12, LSAT, DSR50, etc... On top of that, I think this game has the highest image quality and best framerate of the series. I've been playing a bunch of previous CoDs in the two weeks leading up to buying this one and it's startling how smooth an experience BO2 really is.
The sad and frustrating part is that we're still saddled with the awful MW3 netcode, where some dudes are just straight-up immortal and enormously obnoxious. Having a lot of practice with MW3 has prepared me though and after a rough first couple of days, I've got a handle on it and can deal with it most of the time. Treyarch's patented "play the same map over and over AND OVER AND OVER" technology is kind of a downer too. I've played like 10 hours of Hijacked but I've played Overflow literally twice. Twice!
As for best scores, I still haven't graduated out of running like care package/guardian/sentry and I only hit those maybe once a match, but I've had a few 40-3 type scores. I've had some fun kills like a SMAW quad, R870 triple and I got a DSR50 cross-map noscope through smoke on Carrier for the lulz.
Oh, is there like an easy overview list of weapon changes? I remember people praising the PDW when the game came out, but I've found the Skorpion and maybe MSMC to be the best SMGs. Same with R870, but I'm doing a hell of a lot better with the S12 and M1216, while the 870 feels like it's shooting puffs of air. I had SIX hitmarkers on a dude in one of the two matches I tried using it before bailing on it. The S12, on the other hand, is already on second prestige.