G-Tac Pro seems pretty interesting... I had been tempted to write an app like that myself for some time now, because nobody seemed to care enough to log GPS + accelerometer data. Their website really doesn't tell me much though:
http://www.libertyforone.com/g-tac/g-tac-pro
I would be most interested in what format it exports the GPS and accelerometer data in, because I don't think you're really going to do much useful analysis on the iPhone itself. But if you could easily dump the data to a computer and analyze it there, then you might have a decent learning tool, rather than just a neat gadget for show. Obviously, it doesn't have the precision, accuracy, or array of features that something like a Traqmate does, but for $20 that could be a really sweet deal (the cheapest Traqmate is $699 MSRP, or $600 if you get a good deal new), and you could potentially do neat things with the data like video overlays.
I'm somewhat tempted to just buy it and mess around with it, but would really prefer more information first. If it does what I'm thinking it does, I could do a side-by-side comparison with Traqmate in my car (I just finally ordered one for myself), except that I still have 4 weeks until my first race this season...