I have the Polar 55 and the RS800cx. Both would need Polar to change the battery - but I've had the P55 for three years now, and it hasn't needed a change. The RS800CX is more recent, and a more expensive model. What I especially like about the RS800CX is that I can use it biking, and get speed/cadence measures, and then upload the whole thing to Polar's trainer software. It also has easy-to-use lap timers, exercise labels, and so on. The software lets you plot graphs to show how hard you are working out, at a very fine granularity, and I've had fun overlaying graphs of, say, Insanity Plyo over KickMax. It's instructive to see how the calorie burn rate is during the workout, and how high/low the peaks are, and how long recovery takes. I especially like the ability to store many exercises on the RS800CX before uploading, which is handy on trips. I go back and label each workout with the specific dvd or activity. Uploading is very fast and very simple.
Both models have a chest strap. I don't notice the strap any longer. The RS800CX has a newer version of the strap that is slightly more comfortable, but really I can't tell.
The single thing I don't like about the RX800cx isn't the unit itself, but the upload irda that I bought with it. It's frankly an expensive piece of junk. Many, many people have trouble getting them to work - the positioning is very picky. While I managed to get mine working, the slightest tap caused it to fail. Fortunately you can get other connectors which are more forgiving (and which cost far less than the one from Polar). I replaced mine and haven't had a bit of trouble since.
I have also tried the Suunto high end HRM - but I didn't like it. The unit itself is filled with features that I wanted, but it's not even slightly user friendly and I found myself having to fuss too much with buttons when I wanted to change something. Very annoying and distracting. While a person can get better with practice - why pay that much for a unit that's going to be a pain? Also, the software that came with it is also nowhere near as user friendly as the Polar software, and the customer service is not up to snuff. I returned it, and went with the RS800cx. The thing I kind of regreted about returning the Suunto is that it calculated EPOC, but it looks to me as though I am getting very similar information from the Polar when I look at recovery rate, or if I leave it on for half an hour after finishing a workout.
There's also a Garmin that many people really like, similar to the high end Polars. I haven't tried that one.