I use a Polar one aswell - F4 black thunder (!) - cost about £50.
Its completely reliable, but has a few annoying lack of features, eg you cant pause a workout, so if you stop at a tea shop (cycling!) you either sit there with it beeping or stop it, but then you need to note down the stats because it only records one set of stats at a time, so once you start it again you lose the old data. You also cant change the settings mid session eg toggle between heart rate and percentage which would be useful sometimes. Juts little things that I never even thought of before I bought it. I dont regret the purchase, but I'm now looking at some of the Garmin GPS/HRM units, but mostly because I want a cadence sensor and dont want multiple units.