imho give or take a little, it's as stable as the next language/framework. I've used java and python (and some of their frameworks), but then again I've never really done the core sys admin for any of the stuff I've built. It's not perfect, but it works. I think as long as you have a good host - you're golden, if not you're screwed.

no, it's not running on a cluster yet - we're not running into performance probs yet (though it would be a nice one to have if our numbers go high enough)