Well I've had several cheap ones - and very few things in life have given me so much grief. My lidl wired one worked well at times, but it let me down on two of the biggest runs I have ever done. I'm not looking for a great deal of data, but I want it all the time. I then moved to an Aldi wireless one with Heart rate, or more precisely 3 Aldi wireless ones. The first one never picked up the magnet whatever I did. The second one worked for a while, although it cut out when I put my lights on, the Heart Rate was much more volatile and inconsistent than my Polar HRM and it eventually stopped reading speed as well. The third one is in my box of bike bits unopened.
I was one of these people who was lucky enough to get a Garmin 500 from Best Buy for £100, and it really is pretty much the best cycling buy I've made to date. I record every mile, both commuting and on my road bike, and have easy access to it all. Santa is bringing me a cadence sensor. A year ago I would have thought a gps based computer was overkill for my needs, now I wish I had done it sooner rather than all the frustration of trying to make the other ones work. I suspect many people are the same, they'll start cheap and sooner or later land up with a Garmin.