No, I've never had a crack with a pair of mustangs. They're NZ$700 or so a pair, so not cheap. From a friend that's swum with them, the biggest problem for him with the footpocket was that like a normal shoe, it's most comfortable with the foot in the neutral position, rather than the fully extended position that you kick in. Opinions on the footpockets differ, but people tend to love 'em or hate 'em.

I personally think that the andronov-style footpocket, if custom-made, is the best there is for power transfer while still being comfortable enough to play in. If you are interested in playing around with the mustang footpocket, contact C4 and see if you can buy a pair of footpockets only.

ps. For the love of all that's holy, don't try and buy from that shop I linked to. You will get shafted.