Would love to know if there are any known compatibility issues yet. If I understand correctly GXT 3 has moved in a direction so that it integrates better with GWT 2.4 by using more of the core of GWT. Might run an experiment. It says in the setup guide; "Note: GXT 3.X requires GWT 2.4+" so my guess is it should be fine. There are some nice new features in the release notes, like Super Dev Mode.
"I test about once a week with GWT trunk, and have had no issues either with the build or with using the app. I would anticipate that will we officially support GWT 2.5 when it is released, but there are almost certainly aspects that are not completely tested yet, and cannot be until 2.5.0 is finalized.
The GXT 3.0.0b build should work out of the box with GWT 2.5.0-rc1 - if you have any issues, please let us know."
Once you get past the setup details of Super Dev Mode, it does indeed work with GXT 3. That said, I still find the standard dev mode preferable both in terms of setup, loading time, and ease of debugging, though I suspect this may change as more tools support it directly.
I have developed one Grid component using GXT 3.0 UiBinder with GWT2.4, that worked fine without any issues. After upgrading to GWT2.5 the grid is not functioning properly like it is able to retrieve the data from database but it is not showing the rows in the grid.