Heart rate monitors come in 2 varieties
Timex T5J031 Unisex Digital Fitness Heart Rate Monitor Watch - I bought this one and after 3 months of usage, I have been very happy with it. However, I'm a first time user of heart rate monitors.
Polar F6 - best rated on Amazon, but the battery is not designed to be user-replaceable
Omron HR-100C Heart Rate Monitor - Best price/performance ratio, but has issues:
This monitor seems to require just the right conditions to work. When I used it on an elliptical trainer (the Gazelle, whose own pulse monitor seems to show double my heart rate most of the time), it gave believable readings, not too far from ones shown on an Omron sleeve-type blood pressure monitor. But when I used it on bike rides, or just walking around, even after wetting the chest strap (not easy to do constantly when you're out in the park and its cold or you don't want to pull up your sweatshirt in public!), it gave very weird readings, when it gave any (half the time it just says zero): a rate of 67 when biking up a steep hill, and rates of 120-160 when just standing next to my bike, relaxed and admiring the views from Central Park! This happened so much I gave up and returned it.
-- http://www.amazon.com/review/R1HNQ8QA1PYYZG/ref=cm_cr_rdp_perm
Well, I discovered why it didn't work very well. The battery in the watch died after just 4 weeks of use (about 15 workouts total). I know batteries die, but I just got the thing and the monitor's erratic behavior didn't win me over even if it was the battery.
-- http://www.amazon.com/review/R1XIPTC8VS15YT/ref=cm_cr_rdp_perm
I used to have Polar hear rate monitor, but decided to try this one since it was only around 35 dollars. (So I know how to use one correctly.) Unfortunately, the heart rate portion is highly inaccurate. It often doesn't read my heart rate at all, and when it does is hit and miss. Sometimes it only is reading every other heart beat, so it will read "85 bpm" when it's really around 150, and then for a minute or two it will be accurate.
I really wanted this to work, but sorry to say I wouldn't recommend that anyone buy this product for the heart rate monitor portion. I would stick to the Polar heart rate monitors; they were more expensive but they worked like a charm.
-- http://www.amazon.com/review/R1MGYSP86WJDVJ/ref=cm_cr_rdp_perm
"The batteries are trial batteries and are only meant to last for about 6 months. The batteries are not covered under the warranty, therefore, we do not have them so send out."
-- http://www.amazon.com/review/R3AO5824Z2KQCQ/ref=cm_cr_rdp_perm
This watch had all the features I was looking for and worked fine until I tried using it while on the treadmill. Like another reviewer mentioned, it is easily affected by gym equipment and couldn't get a heartrate then. I returned the item (Amazon was great) and got the Timex instead. More money, but it works fine.
-- http://www.amazon.com/review/R1U5A0OVPEUUNR/ref=cm_cr_rdp_perm