I've tried a number of the iPhone bike-computer apps, and blogged about them at length. Reader's Digest version:
- Motion-X: avoid. For GPS otaku only.
- GPSies: deletes speed data. Huh?
- Rubitrack: Integrates nicely with their desktop app (which is very nice), and is nice enough on its own, but nothing very innovative. There's a more limited free version that also integrates with the desktop app.
- Cyclemeter: Very slick. Has a nicely implemented ghost-riding feature and audio announcements so it can be useful in pocket mode. I did get some anomalous speed readings on one ride (421 mph, woohoo!) with this, but I'm not sure where the blame lies.

I haven't tried b.Icycle yet, and there are a few others that look like they might be interesting. Pedal Brain is what a lot of folks are holding out for.

My battery life has been variable, but it seems like a ride of 2 hours leaves me at 50%; an external battery or battery case would be an excellent idea (you can get one cheaply at monoprice.com)