I recently bought and hacked a PSP and am currently playing Crisis Core.
If you play it for the story, it's pretty cool-it has a lot of nice references to stuff in FFVII, you get a closer look at the Shin-Ra corporation, etc. Some things don't make sense...(but I haven't completed the game--about to go to Nibleheim...)the missions are really annoying if you're a whore and want to try and complete all of them like I'm trying to, and the random encounters get really annoying.
The graphics are a bit blocky to me. Midgar seems unfinished, and the invisible walls are frustrating. I attribute a lot of non-exactness to the difference of time between Zack and Cloud's stories but...it's still a slice of nostalgia. I can't wait to see the transition of a certain character falling into madness...unfortunately, I'm going to have to wait ten years after I clear all of these damned stupid missions.
As for the levelling up system, it can definitely get annoying at times, especially when you're trying to level up certain materia and not others.