I don't use a bike computer. For most of my routes, I can get a shortest mileage value out of Google Maps, so I use that. If I'm following the suggested route, it will be accurate miles, and if I'm veering off route, chances are I won't be adding serious distance. I do keep rough track, because it's nice knowing how far I can go. But I don't see the need for a computer, since it's a bit more of a temptation for a thief than my headlight and taillight.

If I want a speed estimate, I ride a known route and time it. I can pick different length routes to get different sorts of estimates. A short fairly flat 2 mile route is good for a top speed estimate. A long hilly route would work better to find my average speed.