i have the polar f6. i'm pretty happy with it. it's got everything i need, visual and audio HR zone alarm, it can display my exercising HR either in bpm or % of maxHR. it gives me my max and average HR, time exercised and calories burned at the end of each exercise session and stores like 20 sessions in the memory. it probalby does more than that, that's all i really use it for. there are other models that do more than that, some that you can even sync up with your computer to really analyze your exercise data.
i think polar is probably the best as far as HR monitors go, but i think enough competitors have joined the market in the last couple years that i could be wrong. they are quality though.
oh, if you do get one from polar, get one that comes with a replaceable battery in the chest strap. mine originally came with one that didn't, so when that battery died, my only options were to either send the strap back to polar where they would send me a new one (for like $20 or so) or buy the one that lets you replace the battery (which was like $60). i chose the latter since it'll be cheaper in the long term and less of a hassle.
http://www.polarusa.com/products/fseries/f6.asp