As others have said, it depends on what you want it for. I use my Garmin Edge 305 for many things and can't imagine not having the features it has (even though I want to get the new 500 now ).

I use it for speed, average speed (a big deal me ... I like to know my average pace at the end of a routine route to see if I did it faster), cadence, total ascent (how many feet you have climbed ... another big one for me), grade percentage (I like to know how steep certain hills or descents are) ... and I used various "lap" fields. I hit lap at certain places and can compare how fast I did it, and at what average pace I did it to other times I did that lap, which might be a hill or one long stretch of flat before a hill or whatever. And another field tells me the distance of that lap. I LOVE the lap feature. Comes in handy for things like intervals, too, if you want to compare how fast you did today's 2 minute intervals to last week's two minute intervals on the same terrain.

Once I started using the lap feature, I sometimes cared less about over all time and pace, which can vary with traffic, signals, people you're riding with ... and focused on building speed on specific areas of a route, namely the hills. I can still feel good about my progress when I see that I climbed a hill faster than I ever have before, even though the group I rode with was slower than usual, causing my over all pace to be slower than usual.

So, again, depends on what information you want to know on your rides. And sometimes you don't know what information you want to know until you know what kind of information there IS to know!