For various reasons, I've never had the pleasure of delving into mod_perl. Typically, it's been the insistence of the company that because we run Windows and IIS, mod_perl is simply not an option for us. We gained some performance benefits with ISAPI, but it just wasn't good enough. Now, however, the tide is changing. We're switching over to Linux/Apache and mod_perl is our future.
Interestingly, though, we still do much of our development on Windows. Naturally, I was worried about mod_perl on Windows, but I gave it a shot. I couldn't believe how easy it was to install (there's even a ppm!). I couldn't believe how easy it was to configure. I couldn't believe how much faster my test sites ran. I couldn't believe how fast they crashed and burned as soon as anyone else tried to access them.
Also, seeing the following in my error logs was more than disturbing:
Unbalanced scopes: 45 more ENTERs than LEAVEs
Unbalanced saves: 382 more saves than restores
Unbalanced tmps: 121 more allocs than frees
Unbalanced context: 36 more PUSHes than POPs