I've been using this Garmin Forerunner 405 watch to track all of my rides. I decided to go with this rather than a traditional bar/stem mounted computer because it's easier to swap between bikes since it uses GPS rather than a mounted sensor. It's a running watch, but it has a cycle mode with a lot useful features. I bought mine from eBay for $150.
Beyond tracking how far or how long I've gone I learned how to track how hard I've gone. The built-in heart rate monitor, which is probably the most important aspect of training, gives you a number of ways to show how hard you're working. Heart rate monitors are great because they aren't too expensive, but a clear understanding of heart rate zones in conjunction with a rated perceived exertion scale (RPE) and a solid year-long plan is necessary to get the most out of it. The next best thing would be a power meter, which calculate wattage output, but they can cost well beyond a thousand dollars.
The bezel surrounding the face is similar to that of the ring of an iPod. It's touch capacitive, while not as sensitive. It's pretty easy to cycle through customizable screens while riding. Above is the watch with the handlebar mount. It's rather bulky but super easy to mount. It came with zip ties, but I just cinch the strap of the watch down to hold it in place.
The watch also features a cadence sensor and speedometer (sold separately), but it's worth it's definitely worth it to me since I use a trainer and GPS is useless if you're not going anywhere. My only gripe is that it uses zip ties to hold everything in place. It starts to look rather clunky, but gets the job done.
The Forerunner isn't perfect. For normal rides it's great, but sometimes abnormal happens. It holds a battery charge of up to five hours. I often have to charge it every other day. Sometimes my rides spill over five or more hours, and I end up calculating the halfway point to get my total distance. While Forerunner is water-resistant it sometimes has a mind of its own when a drop of water falls onto the touch sensitive bezel.
On longer rides I usually carry my iPhone with me in case something happens. Garmin just released this great app called Garmin Fit a couple of months ago. You can even get an adapter so you can use your heart rate monitor strap. I use this app whenever it's a long, dry day of riding. I've used the Nike+ app to track my rides before I got my Garmin, but on rainy rides my phone starts acting up due to moisture. Also, you can't view your stats as you ride unless you got one of those ridiculous looking iPhone mounts.
What I love most about the whole Garmin setup is the website it wirelessly backs up to. Garmin Connect, out of all the ride tracking sites I've tried out, besides Strava, has one of the better interfaces and tools. Here's an example of a ride I did the other day. It's very clean, detailed and provides an endless amount of information. The visualization of information is nice, too. It integrates your choice Google or Bing maps, which is useful for sharing your route or planning new ones. You can even program and upload customized workouts to and from your Garmin device.
Don't get me wrong, I'm not entirely consumed by numbers and results, but it's been harder for me to walk out the door without my Garmin. For me cycling has gone beyond pure recreation and fitness into something more dedicated and serious. In its own twisted way it's become more fun, reaching the outer boundaries of my own limits.