No reason to hate it, but ...
I can understand people having a small annoyance with Growl. I have to admit I'm in that camp. It's not that I don't appreciate the work that went into it.... but I simply find it to be something that should be part of OS X itself, vs. a separate app. All it does is allow other software to generate different styles and configurations of on-screen notifications.
When the app was completely free (no compiling necessary), at least people felt like they had nothing to lose installing/using it -- so it really caught on. But as soon as the developers started asking for money for it, it woke a lot of people up to the fact that really, Apple should have just bought or licensed the whole thing and built it into OS X. It would have gone well with Lion's release, actually. (Just tell everyone else to install an older version for the same functionality.)