I've been using GSB 2.9 for about a week now, and am curious as to whether other people are experiencing the same issues, and/or whether any of this has improved with GSB 3.0 (or perhaps a later edition by the time you read this).
Most things work pretty well. The Market is one of the most troublesome areas. E.g., some downloads and upgrades hang in the middle. I've resorted to upgrading apps individually rather than using the "Update All" option since the latter seems to queue up too many requests and is likely to bog down my Eris. Keeping the phone awake by jiggling the trackball seems to improve the success of downloading individual apps and/or updates.
Installing large apps from the Market (e.g., HandCent SMS and Flixter's Movies) can result in "installation error: insufficient storage available" messages. This is despite my having installed Cache2Cache and my heavy use of Apps2SD. I have nearly 1GB free on my SD card and Settings->Manage Applications reports that I have 99MB free out of 160MB. I was able to get both of these apps to install immediately after a fresh reboot (one reboot for each app).
Moving apps to the SD card fails intermittently (~20% of the time), but sometimes this can be overcome with a combination of persistence and reboots. E.g., I had a lot of trouble getting "KJV Bible" to move to the SD card, but after ~5 attempts was successful.
The battery life and responsiveness seems a bit better than under KaosFroyo 37, and I'm able to install more apps than before.
Again, thanks in advance for your feedback regarding your own experiences.