I stuff my Xootr Swift in a hardshell suitcase and only ever pay the regular bag-check fee. since the bike + suitcase weigh less than 50#, there's no weight surcharge. no one has yet asked to see inside, either, since it looks like regular luggage.

that said, they do treat it like a regular suitcase, and I have winced when seeing them toss it on the conveyor belt. I can see that gate-checking a smaller-folding bike would be preferable, though I might prefer not worry about the potential TSA drama