I use cyclemeter:
http://www.abvio.com/cyclemeter/ . No problems, except for that my iPhone 3G runs out of memory sometimes and crashes (happens with other apps, too. Worse on GPS ones). No relation, just a happy user.
Cyclemeter doesn't have quite the 'social' aspects of iMapmyride, but it does do stuff like post twitter updates of my ride with links to the map. Funny story: my wife actually came to pick me up even though I didn't call her because she saw the tweets that I had stopped in an odd place for > 10 minutes (I had some problems with a tire patch).