Haven't tried one, but IMO the GoPedal seems kind of useless. Simplified, all it does is amplify the value (think 1-100) that the pedal sends to the throttle. You put 20% on the pedal, the throttle thinks its 40%, and so on. Obviously it's probably not a linear "boost" but you get the idea.
Personally, I really enjoy stepping on the pedal all the way and still being under the speed limit. How many cars can you do that in? Also I have to imagine that amplifying the throttle makes it harder to maintain a low speed unless it's somehow damping the throttle rather than amplifying it.
Also, the GoPedal is a higher price for a reason. The Smart's codes (OBD, etc.) are not standard. On universal OBD readers my 451 comes up as a BMW. Chances are the GoPedal, Sprint Booster, etc. use certain codes.