This controller is better than Logitech's Dual Action USB, even if you ignore the added vibration feature. The area under the four main buttons has been hollowed out, making it easier to hit them, and all the buttons have a slightly higher-quality feel.
The biggest improvement is in the D-Pad. The D-Pad of the dual action is terrible, which makes it unusable for many types of games. For example, it is impossible to reliably use even simple fighting game moves such as SubZero's freeze or Ken/Ryu's fireball. The Rumblepad's D-pad, however, is much improved, plus it has a useful button that swaps the D-Pad and the left analog stick in case your game doesn't recognize the D-Pad (some don't). The D-Pad feels a little strange, and it doesn't quite reach the quality of the D-Pads on Nintendo or Sony products, but it does the job.
I have this pad, a Logitech Dual Action, and a Saitek P880, and I recommend this pad as the best of the three (unless you need six thumb buttons or a shift button, in which case Saitek is still a good choice, with or without vibration).