I'm only a casual user of PPI via other things that use it, but I'm extremely glad for its existence. In particular, better editor support is critical for perl right now and PPI goes a long way forward on that front.
Excellent concept, superbly executed and maintained.
Recently I had to add the /m flag to all regexes in a program (ghc-asm) because Perl 5.9.x removed support for the $* variable; PPI solved this problem in 5 lines. Without PPI, it would take much more work, and the result would be much more fragile.
I've been using PPI for doing the parsing of Perl in CPANXR and I must say that I'm more than satisfied with the quality of this module. The saying "only perl can parse Perl" is still true, but PPI comes in close (and hey, PPI is pure Perl). I've only used the tokenizer part (PPI::Tokenizer) so that's what I base this rating on.