I have a Polar RS400 which I cannot be without. Ever.
This is the one I have and I can't imagine ever doing any workouts without it. I still have a laptop running XP for the software that came with it. I hope they never discontinue it. I have no interest in any of their newer offerings. DH has an RS300 that he uses everyday.
DirtDiva -- I also have an RS400 and wondered if you've had problems going through watchbands? I'm on my third. My sweat just destroys the rubbery part to the point that it eventually falls apart. You can only buy a replacement band through Polar. Because of the odd shape, generic watchbands won't fit. I love my RS400, but if I could change one thing about it, that would be it.