My cycle computer is one of the best investments to my moutain bike. I do a lot of trail exploring and marking and distance helps a lot, the wireless system dosn't get caught in twigs. I use the constant speed as a very good guide when climbing hills. Including a clock and a timer its a handy thing to have, and they are cheap to get. Average speed is the only thing i dont use because its just a stupid principle when i'm constantly stopping / starting etc, climbing and decencding.
I do. I have the Cateye Enduro 8. It does have a wire but this one is built tougher than your normal bike computers as it's meant more for mountain biking. Has a tougher wire. Allows you to keep two tire diameters. That's perfect for me since I switch between slicks and mountain bike tires
I remember getting it on Ebay for pretty cheap. Looking now I found a bunch for just about $15 USD with BuyItNow.