I was frustrated by the splitscreen co-op. Here I have this nice big LCD TV, that I bought so my games would look nice and pretty, and my partner and I had to sit not 5 feet away from the screen to see because the splitscreen made it so small. It didn't make sense why Capcom did it this way, and I feel it would have been a lot more enjoyable if they took a more traditional approach to the display.
Online co-op though, was awesome. Playing the first time through was a little frustrating because the AI was sometimes lacking, but having a real person there to make up for the shortcomings of the AI was great. The inventory was a bit bothersome but it was manageable. If I had to name one thing I didn't like, it would be that communication was limited if both parties didn't have a microphone. It may have just been me, but I felt that the existing interactions felt awkward and obtuse.
Quick time events can be a bit more painful with co-op, but its part of the game. If you get lucky and find a good partner online, the game is all kinds of smooth.