I drove to Louisiana last year with a friend who had one of these. The biggest hassle was having to frequently retune to a different frequency as we traveled. The sound quality wasn't that great, and the constant retuning was irritating.
What year is your Miata? If it is new enough to have a factory head unit that can control a tape deck or CD changer, then you should be able to hardwire an MP3 adapter to it. Much better solution imho. I bought am AuxMod for my RX-8, and I've been very happy with it.