I think the CS300 is the only Polar one designed to be used on and off the bike, with bike readings. It's possible to get bike mounts for some of their others, but they're just HRMs.

The Polar strap does need to be sent away to replace the battery, but on the other hand, they're usable in the pool, and I think that's one of the reasons.

Suunto and VDO both have models that can be used on and off the bike, but they're much more expensive than the Polar.