No matter what you do, you have to flash the altell ROM first to upgrade your radio to support GPS, Rev A etc. If you stop there, you'll have a nice, stable, factory-like phone. Nothing fancy, but it works. Instructions for that upgrade are here... be sure you don't screw up step 8:
http://forum.ppcgeeks.com/showthread.php?t=24979
From that point, you can go on and flash most of the custom ROMs that people have cooked up (as long as they aren't carrier specific). These give you all kinds of customizations, tweaks, enhancements, etc. However they also introduce risk (each time you flash your phone is another opportunity to brick it) and sometimes stability issues (OS/application crashes). It's a matter of preference how far you want to go...