I've been using lighttpd as a development webserver for an scgi based webapp I'm building. I was very impressed with its ease of configuration and automatic load balancing between scgi processes. Getting
mod_scgi to work in Apache was a nightmare.
I haven't encountered any problems in my development environment, but it seems that it's no longer in active development (last release was August). The email list and forums are generally dead. Although this isn't itself evidence of a problem, inactivity seems to raise questions.