I dont own Tony Hawk myself, but I know 4 friends that do have it. It is VERY similar to the PS2 version graphic-wise. One major difference is the psp version doesnt seem to have shadows on the characters at all, that is the only major difference - besides that, the textures are just compressed a bit more. The PSP version DOES have 4 more levels than the PS2/Xbox/GC version though.
The only other difference is the control setup. The PSP is missing the 2 other shoulder buttons so they improvise. The 4 face buttons are the same (x = ollie, o = grab or flip i forget, square = whatever o isnt

, and triangle = grind). 2 big things that i know bothered some at first were 1) the L shoulder button is the "get off your board and run around" button. I am used to always hitting L and R because on the consoles that flattened out your landing....so i hit it a lot by accident. Also, 2) you MUST use the d-pad since the analog stick is used for camera only.
I know a few people that were really angry when they found out that you couldnt use the analog nub for movement, but everyone got over it quick, and it is played all the time wirelessly around me here.
hoep that helps, if you need any more info ill try to help