The Mach Bike is irritating to use, besides when you need it (or if you're doing time challanges on Cycling Road). I prefer the Acro Bike, because it's a lot easier to use overall, and it actually makes going places faster, because you're not hitting everything (I'm not all that great with the Mach Bike, except on the Cycling Road, which I've gotten the path down pretty well now, so I have great finish times).
Really though, with the introduction of the Running Shoes, I don't rely on the bikes nearly as much as I did in R/B/Y or G/S/C. It's my preferred method of travel in the Advance Gen.