I ordered my iMac now deliberately so that I could get it with Mountain Lion. A new OS is always going to be buggy, and I'd rather have a fairly stable OS to begin with, and then I'll upgrade to Mavericks once it's been around for awhile.
Hmm... this is a good point. New OS can be buggy. However, I have Mountain Lion in my App Store account so I could theoretically downgrade if I really needed to.