Well many people are curious if SystemTap is ready for production. On last UNIX DAYS we had an presentation about SystemTap prepared by our friend. Well during his preparations for the conference he almost got used to many system crashes a day. That alone speaks for itself. Then SystemTap currently lacks user space tracing and many many more things. I wouldn't put it in a production anytime soon and I don't know anyone who is using it.
James posted a well balanced blog entry about it.