My problem with the prok 2 is that it seems extremely dispoportionated. There are some beautiful moments, but I never feel compelled to listen to the entire thing through, which I've only done a few times. It's a massive, and at times awkward piece (but perhaps this is due to the difficulties?). I've heard renditions of the cadenza that made me want to vomit. Of course, when the orchestra comes in afterwards, it really sends chills down your spine.
But I prefer his 1st and 3rd concertos. They have a much more satisying form for me, they propell you from the beginning of the 1st movements all the way to their satisfying conclusions. And what an ending that 1st concerto has! One of the best out there, I think.
As for his 4th and 5th concertos, they are very odd works that haven't quite grown on me yet. Interesting for sure... but strangely unsettling as well.