I have owned a POTO for ~4 years; bought mine without ever having played it because my wife and I love the theme (and the price was good).
The trap door seems to be the most impossible to replace part. The trap door on mine was 50% in tact when I bought it, so I had to hack it a little, but it's fully functional and pretty good looking. Ramps and plastics seem to hold up well, but I believe there are new replacements available if necessary. Also, the Phantom's mask, which is printed on the *outside* of the backglass, is very often flaked away. However, my wife printed out a replacement mask and stuck it to the backglass, and it looks great. A 10 cent solution that did a B+ job (i'm sure with some experimenting with different paper types etc you could make it an A+ job). I have a hi-res file of the mask that I could share if needed.
Gameplay wise, it is simple but challenging. The main goal is the multiball (which actually has a pretty interesting progression). What keeps me coming back is the difficulty of the game. It is very fast and unforgiving. The super jackpot is a serious achievement. It's a great game to compete with friends on because the ball times are short, the objectives are clear, and getting a good score is challenging. I'd be wary of having it in a small (1-3) game collection just because of the simplicity, but it works great in my 5 game collection.
Finally, what most people say about the game is that the artwork and music are its best aspects. I'm more concerned with playing the game than looking at it, but I do agree that the sound is fantastic (I hooked mine up to a sub and it's incredible), and the artwork really is beautiful and so detailed.