As a current ITC 510 racer, here's my two cents:
Pros
blast to drive, pretty forgiving, great handling, fun to beat up on "faster" cars in corners
Cons
slow straightaway speed, parts can be a challenge to find (but not impossible), limited future as IT car, few cars in class
510s are slow compared to just about everything else out there. It is frustrating to have faster Miatas and others beat you down straightaways to a corner only to hold you up through the corners. The fact that they are older cars means things do break more often and when they do, you need to sometimes get resourceful in finding some parts. Overall though, I really enjoy my 510. The parts issue isn't that big of deal. I have yet to need a part that I could not find.