I'm thinking about getting a cycle computer. What should I look for?

I commute 6 miles each way, about 3 days a week, mostly on a paved MUP. Starting to venture off-trail a little bit for errands. My bike: a 700c Trek 7000 hybrid.

A friend gave me a used CatEye, but the plastic that forms its mounts had grown brittle with time and snapped apart before I could install it. Can't really use it unless I can rig up a dependable mounting system, which I don't think will be likely.

I've never used a cycle computer. What features have you found helpful? What doesn't matter, or what should I avoid? I do commute after dark this time of year, and I understand backlights may be hard to find.

How do you use your computer?