I'm using a 4850 and everything seems to be ok, although since this whole thing is still beta and I've never used an "officially" supported card, it's hard to tell if the instability is due to the card or the SDK.
With the current infantile functionality of the SDK and the difficult time AMD seems to be having getting this thing to official release, it will be interesting to see how much longer they support this SDK and push to be a player in GPU computing.
I have no doubt they're (AMD is) committed to GPGPU processing. That had to have been one of the key motivations behind the acquisition of ATI. But I wonder when Brook+ will be supplanted by OpenCL. And just how closely they will resemble one another.