One of my old hardtail MTB that I have a Tourney RD. It has the little wheelie you talk about. That thing shift so bloody well that I have no compelling reason to "upgrade". I dare say it shifts as good as my rd7800 unit on my roadbike. I think alot of it has to do with proper cable/derailleur tuning.
It is a chunky unit though. You can see the reviews over at Amazon.com