I won't lie, I haven't found a single product I've bought with the Gamestop label on it that didn't do well for me. Also, as far as rumble function goes, it'll definitely be on the package for a serious sales pitch... it's a high-selling function. You'll especially want to buy one that is 2.4Ghz, they are found to have less 'lag' that even the Wavebird is known to have around specific areas with common RF interference.
Don't let this fool you, however, the Wavebird is one sweet item. I've had mine since they were released and it's flawless. If you want a rumble feature, there are 2.4Ghz products that include this, as well as programmable 'hot keys' you'd probably be interested in instead. Honestly, although I'm happy with the Wavebird, there are controllers out there with added functionality you'd probably want to consider having, especially if you're wanting one that rumbles. The Wavebird if a great product, but it's just wireless, nothing more than that. Go to
www.google.com and do a search on '2.4Ghz Gamecube Controller' or something similar and I bet you'll find what you are looking for with more functionality than the Wavebird. It all comes down to what you want in the controller, good luck and I hope what you decide pays off for the long run.