I've been playing this since the the beta launched with a few friends. So far I've really enjoyed it, there are few games that properly execute co-op.
The difficulty I believe is one of its biggest problems at the moment, Overkill simply isn't challenging. The first game's Overkill was much more difficult than it is in the sequel. This may be due to the fact that shock troops aren't nearly as threatening, and snipers only serve to trickle you down. The lack of "Sam Fishers" doesn't help either. Being able to rush maps such as "Ukrainian Job" on Overkill in less than 35s is ridiculous.
Overall I feel the game is an improvement over the original, most importantly with regards to the mechanics and gun play, however difficulty isn't one of the improvements.