Bruce Byfield has written a pretty good review of Ubuntu 8.04 Hardy Heron's place in the market. He concentrates mainly on the tension between "free" software and proprietary extensions. For those of you who have wondered exactly what that argument is about, this is for you. Byfield lays it all out succinctly and accurately.
I'm an Ubuntu fan. I'm a Linux and free software advocate. There are problems in paradise, though, and Byfield has laid them out in an easy to understand form. Well worth your time.