I've been using SUSE Studio since I first got my account and I love it! I've built two projects with it already, and I expect to build more. The only issue I have with it is that the pre-defined patterns in the package selection menu, at least for openSUSE 11.1 appliances, are different from the same patterns in a fully-installed openSUSE 11.1 system. That is, if I select the Kernel development pattern, I get fewer packages in the appliance than I would if I selected that pattern on an installed system.
In any event, if you can get an invitation, I highly recommend it.