There are small quirks here and there, but mostly, my transition to 7 has been flawless. By quirks, I mean;
-Chromium/Chrome does not work
-Skype needs the beta version, which really isn't a very transition friendly edition.
-Older Game installation issues (compatibility mode works for most, but not all)
7 is amazing, I don't know why many are complaining why Flash doesn't work, cause I stream flash via youtube flawlessly. Although this might be because I'm using Firefox 3?
Thunderbird works fine as well, so no issues there. MSN, Garena, Punkbuster, uTorrent, VLC, Winamp, cpu-z, Everest, gpu-Z, and 3D MARK work fine. 3D MARK might show improved scores due to the more optimized OS, and games feel 'smoother' or might just be a placebo effect on my side.
A small issue with software was trying to register my powerISO copy, it would say succesfully registered, but when i boot up power ISO again, it would tell me I am running an unregistered version.
If by Flash you folks meant running Macromedia err Adobe Flash CS3 then I've no idea, I do not have the 1000 USD to shell out on that particular software

albeit I am inclined to think there might not be any issues.
The reason why 7 ISO might not be working for you guys might (and I am using might really loosely here) be fixed if you burn at the lowest available speed, 4x. Usually this helps reduce burn errors, however, if you are already burning it at the slowest speed them I am sorry, no idea, a bad disk maybe? For the record, I used powerISO.
Is 7 good? Yes.
Would I recommend a complete transition? Depends.
Why? Note that this is a BETA, and it is quite possible that the BETA version is devoid of all the bloatware the RC 1 might bring. For you to completely transition to 7 now, and later find out a rather sluggish (Heaven Forfend) or a disappointing 7 later, it would suck reverting all your data back to your VISTA/XP install. However, personally I am not as finicky when it comes to the OS, and I would gladly use 7.
-HypoG
P.S. This long post is dedicated to compensate for my one liner posts and to avoid my auto-bot posting messages from being detected!