I'm waiting until there is a SP1 RC1 version before installing, but Vista SP2 is expected to be released in final form sometime in the first half of 2009.
I'm currently running the Beta SP2 on my main laptop. I've found it to be entirely stable. I've seen nothing out of the ordinary. But at the same time, I haven't seen any improvements of anything.
So if you're thinking of upgrading, personally, it's quite stable for me, but because of the lack of improvements, I wouldn't say it'd be worth the hour or so to install.
I'm running the beta1 and loving it. It's the best beta1 MS has ever released. No problems whatsoever. Highly recommended. Faster boot up and down time, more stable and generally faster. 3 cheers to MS. see the can do things right if they want too.