I've run the Tamiya 3 speeds and I think it can be fun to shift the gears, but the truth of the matter is I found myself in 2nd gear for just about everything. Once the novelty of shifting gears wears off, you'll probably pick one gear and stick with it. I have a Wedico truck that I intended to build with the 3 speed, but changed my mind. Now it's a single speed, with a planetary gearbox and a 540, 55 turn motor.
My truck isn't done yet, but I know I'm going to like this combination better then the 3 speed that I had in there.