I've used Lighttpd, Cherokee, Nginx and of course Apache (although stripped down for speed) on a high load production site for serving static files and reverse proxying.

Nginx wins no contest. Highly stable, serving for almost 2 years now with zero issues.

Lots of features, much simpler config, uses less resources.

Hot config reloading. Hot binary upgrading.

Low grade? Hardly.

Issues with others:

Lighttpd - configing slightly annoying, uses more cpu and memory

Apache - slow performance and memory hog even when stripped of all it's extra modules, (just try and support 5000 keep alive connections on it)

Cherokee - very nice, well thought out and featureful and fast but had some stability problems in production, I may take a look at it again later on because it has a sweet admin interface