I have a Garmin 305 that I have found really useful in learning new trails and have actually used it to find my way out of an area when I got a wee bit turned around. It works best when mounted to my stem and it has survived numerous crashes. I have the one that includes HR and I have a cadence monitor that I use on my road bike. I have also used it while running and hiking and geocaching. It's a great tool and well-worth the $200 I paid for it. Garmin has their new 705 model out (color screen and other fancy stuff), so the older models can often be found at a discount.

You can set the sampling frequency to every second or every 5 seconds, I believe. The less frequent sampling is better for longer rides and also uploads quicker.