Newer Polar monitors come with the Wearlink transmitter which you can change the battery of yourself - just be very careful when opening the back that you don't damage the little slot that allows you to get a coin in to turn the battery cover (luckily I had another one). Its also a very comfortable strap to wear as its textile rather than the plastic.

My BF has a Garmin Forerunner and I'd never wear it as its humungous. I am thinking about upgrading my Polar S410 at some point this year to one of the Speed/Distance ones: possible an S625x (can use as bike computer too) or RS4000SD/8000SD. I'll wait a bit until the newer models have been in circulation for a while.