I'm only familiar with varnish 2.1, but as to that version I think it's a bit of a stretch to say varnish is well written. VCL is very complicated - just check the request flow diagram [1]. Some of the documentation is very poor - try to find out the properties available on beresp for example (you have to grep the source code [2]), or try to understand the precise function and implications of grace mode, saint mode, or hit_for_pass. The best redeeming quality is varnishtest and some of the other tools that are provided.

[1] http://book.varnish-software.com/3.0/_images/request.png

[2] https://github.com/varnishcache/varnish-cache/blob/2.1/lib/l...