If you want a cheap and convenient method for tracking your speed, distance, highest and lowest speeds, mileage and overall mileage and you are on a budget, then you won’t be interested in spending your hard-earned cash on the more expensive bike computers.
I bought a Decathlon “BTwin” branded trip computer in 2008 and it has been great. The battery is still going strong, despite me using it nearly every working day of the week (so that is almost 3 years worth). Decathlon offered a variety of models and the only thing I would change about the model I bought is, I would buy it with the back light (which goes off as well) for winter cycling when it is dark.
The trip computer only cost me about £14.99 in 2008. Not sure on prices currently, but they will no doubt be cheaper than a lot of other trip computers out there.
It has survived torrential downpours, the type that soak you through to the bone, no matter what you are wearing. It has survived snow & ice and heatwaves. All in all, it is very robust.
You attach a small cylinder device to the stem on the front of your bike and a little magnet to the spokes. The trip computer you simply fit to your handlebars via a seat, so it is detachable when you are out and about. You need to make sure the cylinder device is as close to the spoke as possible so you get a good reading. 5-10 minutes to set up.