I use an old Garmin Forerunner 301 because I'm geeky and I like the data. It incorporates a HR monitor, which is nice in terms of seeing my 'level of effort', and it keeps track of my ascents, descents, speed, pace, and distance. It also has this little 'calories used' number which really makes me feel good (even though I know better than to believe I'm burning 2200 extra calories commuting each day).

At the end of the day, does it get me to work faster, save me time, make my bike more efficient, or otherwise matter? Heck no. But I like knowing, and charting, and keeping track. And like someone above, if/when it breaks, I'll likely replace it.