Strengths: 1. If you regularly run in a built up area then you can forget the supposedly more accurate GPS systems - tall buildings block satellite signals - and getting accurate measurements in the centre of London is nigh on impossible.
The polar S525x does not have this problem.
Ok, you are supposed to calibrate the footpod first - but even if you don't the measurements are remarkably accurate.
2. It's incredibly easy to use - and you can get a tailored combination of stats on screen at any time (I generally choose %HR, time elapsed and distance - but you could go for actual HR, split distance, split time, etc)
3. Unlike most GPS systems - It's not bulky! you have nothing strapped to your arm!
4. If you are considering using online training packages such as www.trainsmart.com - a large number of these packages only work with polars.
5. And finally - its a Polar - should you need a servicing or a new HRM strap you know you are not going to be left to your own devices - a trip to the local running shop should sort you out.
Weaknesses: There are a few weaknesses.
1. If you are a serious triathlete you can buy cadence, power and speed sensors to fit your bike that all work with this device - but I've not managed to find a serious cyclist who has been able to use these reliably. I've had to revert to using my CatEye again.
2. The memory recall is difficult to operate. I now record all my stats directly after finishing a work out, rather than waiting until another workout has been completed.
3. Polar software doesn't work with a Mac! Not the end of the world though - there are a number of internet sites that offer ways round this.
4. Again, if you are a triathlete, there is no quick way of switching between the cycle and run modes - so you can't use this device for recording accurate time / distance for both legs of a triathlon or duathlon (but then again, most GPS systems are too bulky to wear when racing - so offer no advantage over the Polar here)
Overall: I own both the Timex bodylink and the Polar S625x.
My device of choice? The Polar - everytime.
Not only do I know I wont lose a satellite signal mid way through my run (or get interference from anyone else's device), it weighs almost nothing, gives me all the info I need on one screen and is frighteningly accurate.
Polar - you need to make a few tweaks to make this the perfect machine for multi-sport athletes - but you're getting there.