I found Parse::RecDescent nice, as I am fond of recursive descent parsers - the top-down approach of parsing seems elegant to me and it can produce nice, sensible error messages. The surrounding utility functions to automatically generate ASTs etc. are also nice.

That said, I found two showstopping drawbacks to Parse::RecDescent. First of all, it's horribly slow for moderately complex grammars (Javascript in my case). Second, if you have a syntax error in your grammar, the error messages range from obscure to opaque, and you can't go and fix the parser itself, because Parse::RecDescent is written in itself and the distribution (at least at the time when I used it) didn't include the source grammar but only the generated result. From time to time, it wouldn't raise an error at all, or raise an error in the wrong place.

Personally, I recommend Parse::YAPP nowadays for any serious parsing (if it has to be done in Perl). The drawback of Parse::YAPP is that it needs a left-recursive grammar where Parse::RecDescent wants a right-recursive grammar (or vice versa), but the resulting code is nice, the driver for the parser is simple and could be written in XS to get a moderate speed gain.