You can use video in Google Talk, the battery stats are more feature-filled. Supposedly GB has better battery life. Bluetooth stays connected. The screen is much more responsive, though that is not likely a GB thing vs just the work people put into the ROM.
CM7 specifically also has some nice things like the notification power widget. Man will that thing spoil you.
Many of the ROMs also include updated Adreno drivers for better graphics performance. And you generally lose HDMI as the trade-off.
Also, I think you may find over time that fewer apps support Froyo? GB is still by far the most prevalent version of Android, Froyo is getting smaller and smaller. The latest Firefox browser supports 2.2+. How long before it is acceptable to only support 2.3+ for apps?