eB2.1 SP5...we just upgraded to this from eB2.
Could be a relatively painless upgrade or a nightmare depending on what your plans are. We implemented the new .NET UI as part of our upgrade (previously we used GUI locally and Desktop 2.6 remotely). For us, the .NET "piece" caused most of the problems...some our fault but others beyond our control.
If you go .NET, be aware that it's slower. Some of our users lasted about a day on .NET before we reverted them back to GUI. The other big problem we had was the way we connected to the DB's. Unbeknownst to us, it matters very much how you make the connection (direct connect vs using -H, -S, etc) when webspeed is involved. Locking was a huge problem.
Also, there is a known problem with running reports to Page in .NET (you cannot use window or terminal in .NET). Basically, the data cannot be trusted! Users will get annoyed very quickly. My advice...make sure you turn off the beautify reports functionality unless QAD come out with a fix soon. But I don't want to alarm you, as this is a problem only with running reports to Page in .NET. It's not a data integrity issue.
Good luck!