Depends on how tolerant you are of bugs.
Stability is fine, but bugs like black Finder windows when opening your Application Folder, or having to constantly reenter your iCloud PW, is slightly annoying.
It's more stable than iOS 9 is IMO, but I went back to Yosemite and iOS 8 for the time being.
That said, it's not a knock on Apple, it's an early beta and this is to be expected, I'm just suggesting that I'm not sure if it's quite ready for use on an "important" computer that needs to run flawlessly.
That said, El Cap is beautiful and I can't wait for release day.