I have a Polar F4. I've had the wrist unit for years. I recently replaced the chest strap it originally came with (after the battery died) with the new WearLink one--you can replace the battery on those. And I've replaced the battery in my wrist unit twice with no problem.
As far as features, it's a basic model but does everything I want it to. It reads heart rate (displayed as beats/minute or % of max), calories burned, and length of workout. During your workout, you have a choice of 3 displays--heartrate, timer, or time. If you hold the wrist unit close to the strap for a couple of seconds, it will flash to time of day. The workout summary gives you length of workout, time in your heart rate zone, average heart rate, maximum heart rate, and calories burned. It uses your age and weight to help determine calories burned. You can also set your own minimum and maximum heart rate for your "zone."
I have been very happy with this model. If I didn't have one, however, I would probably want the F6 so that it would link to the computer. But I think mine has a lot of life left in it.
