Used a Walmart Schwinn cyclometer for years (the wired type). 10 bucks at Walmart, just make sure you calibrate it (as with ANY cyclometer) so you get accurate distance information. Used it on my one month tour from SC to TX, never had to change the batteries.
I've never tried a wireless but if you do a lot of off-road biking, you might want to consider a wireless as the wire can get caught in twigs and sticks and break off. I've lost one this way.
I use a GPS now.