Has anyone used any GPS, odometer, speedometer type apps on their phone while riding? If so, which app(s) do you recommend? Does Droid or iPhone have any better app than the other? Let me know what a guy's gotta do to get set up...

Or do you think that using your phone on your MTB is just asking to have to buy a new phone?

I guess my real question is: if I want to track my rides and perhaps even my heart rate, what piece of equipment do you recommend? I'd like to not spend a grip, but I'm ready to entertain any suggestions.