Don't put any money on your guarantee.
I've been using DP4 as my main OS since its release. That is, I wiped my MBPs drive and did a clean install of Lion DP4. Daily OS, with all my files, all the apps I use, everything. I'm treating it as a full release every day. This is how you really test stuff. It isn't futzing around with a vanilla copy on a separate drive. Instead, it's about forgetting that it's a preview and relying on it every day while taking your chances. Not everyone is able to do this, but for those that are, they'll learn damn quick whether this is something that's ready for release.
It's extremely stable and there are no showstopper bugs (and my hardware isn't exactly new, either.) This is the general consensus.
If the GM is anything like this (an improvement over DP4, apparently) Lion will be damn-near flawless upon release.
This will be an extremely hard act for the competition to follow.