I get the impression a lot of people aren't that keen on the new main protagonist (Nero) and his story (which is the focus, though you play as Dante for about half the game too), but it's seemed okay for me so far (not like that's the main point anyway, I'm here for the gameplay and insane fight scenes in cutscenes).
As for gameplay I think most agree it's decent but not quite as polished as DMC3. There are a lot of nice new features (Nero's Devil Bringer lets you move enemies around and keep combos flowing, his exceed system lets you add rhythmic inputs to attacks to boost damage and charge up enhanced versions of certain special moves, and Dante can now switch to any style mid-battle) but it lost a bit of flexibility too (less weapons and style abilities in some cases etc.). Not quite as good gameplay-wise as 3 I'd say, but not bad.
DF: So why can a Marauder roll but a multiplayer Turian can't? Delta1-9: You are fighting female turian husks. You have reach but they have flexibility.
I put it a notch below DMC1 and 3. The combat is as good as ever. Enemy design/AI has been improved compared to 3, a bit closer to how good enemies were in 1. The story (if you care about that) is sufficient to keep you going forward.
However it has horrible level design, pacing, backtracking, and recycled content. You have to keep reminding yourself to keep trucking through during the lulls. The fun combat just about holds the game together.
End verdict is to get it nonetheless, and try to get the superior PC version if you can.