I have Cyanogenmod installed on my old HTC Legend (which i use as a phone for festivals) and its great. It has some great features that aren't part of standard Android, certainly breathed a new lease of life into it.
On my Main phone, DesireHD, i run Android Revolution HD, which is customised and optimised for the phone, increased battery life (especially standby) it's faster, smoother, and less force closes on app's than the standard HTC rom.
There is alot to be said for rooting your android phone and sticking on a custom rom, and the XDA forums are a great source of help. Just make sure you read the walk through's carefully and fully before asking questions.