I have a red Polar F4 watch that I got for Christmas. I also have a Garmin Forerunner 305 that I got for my birthday. I use the Garmin for my outdoor runs, but my Polar for my gym workouts (and treadmill runs).
My Polar F4 watch and WearLink Transmitter
I like my Polar F4. It's pretty simple and easy to use, and I think it's one of the most basic of their products. However, it works really well, and I've never had issues with it. You can input your age, gender, weight, and height and it calculates your calories burned. It will tell you your average HR and max HR at the end of the workout, along with the calories burned and workout duration. While you're working out, you can have it set to show either your current HR (with calories burned at the top), how long you've been working out, or the time of day. You can switch through these screens by pressing a button.
I did have some trouble with the plastic HR band that came with the watch. It was uncomfortable, and would bruise my ribs-- especially if I was doing ab exercises or something like that. But then I bought the Wearlink and transmitter, and had no problems anymore. The wearlink is a soft band instead of the hard plastic one I had.
I like my F4, but if I was buying one for myself, I'd probably look at the F7. It's a step up, and it looks like it has some cool extra features. It's not that much more expensive than the F4. The F4 is listed at $99.95, and the F7 at $129.95. I'm sure you could find them a bit cheaper if you shopped around.
I'm very happy with my Polar watch though, and love that I can log my workouts at the Polar Personal Trainer website. I'm huge on numbers (another reason I love my Garmin, too!), and really like seeing all those stats. If you have any other questions about my Polar F4, just ask :)