I bought a second HTC Magic a couple of weeks ago to play with. I hadn't loaded a new operating system on a phone before, so I was being ultra cautious buying a second phone...and even then, I didn't want to "brick" the thing. 

In the end, it was easy, simple and - dare I say it - fun. Definitely in the category of scary until you know how....then you don't look back. 

First I backed up the original ROM from the phone. I can return to a Vodafone NZ genuine load any time. 

I've tried 6 diffferent ROMs so far and all but one was noticeably, seriously, "YES!-moment" faster on my HTC Magic than the standard load. 

The two best are: 

1.  Cyanogen Mod 4.2.5 - for improvement right across the range of function and features. OTA updates provided by "CM Updater"....on-demand, if one exists. 

2. Soulife ADP Remix - based on Cyanogen, but closer to the standard load while at the same time being very fast and including most of the 'good stuf" (wifi tethering, etc...etc...) 

Honourable mention goes to Dwang 1.13. Very fast. Not to many features included, but nothing stopping you from adding them.