I have a GPS and a "regular" bike computer. The GPS eats AA batteries, and a pair lasts a whole day. But of course if I do a lot with it, or used it the day before and haven't changed the batteries ... I'm not going to be able to rely on the thing. On the other hand, the CatEye has something like a watch battery, which I haven't had to mess with since I got it in July.

I like knowing how many miles I've put in my bike ( total, and on a given trip ). I think it's very important to be able to know my current speed, and of course the average for a trip is cool, too. I find this stuff pretty compelling. If you don't see a need to have it, then you probably don't need it. None of this will keep you alive; either you have that compulsion, or you don't.

The GPS is much cooler when you get home. I use it occasionally on very long rides when I get lost in neighboring towns I don't know very well, but it's main use is the data it logs for me. I get charts showing my altitude change, how much climbing I've done, how my heart rate correlates with this and with my speed, etc. And jpeg maps I can email to people who should have come with me. Again, as a confessed geek, I really like having this kind of information. But it doesn't bring about any real changes in my life; I just like having it.