I've used the s625x for a bit over 1 year.
I just had the battery replaced (as insurance against Murphy's Law) before Bayshore marathon. It had no low battery symptoms yet.
Foot pod batteries are supposed to last 20 hours and they do, I replace mine every 3 weeks or so and had only 1 die in the middle of a run.
The Transmitter batteries are user replaceable and probably last as long as the watch battery. I replaced mine when I was having problems but the voltage checked fine and it was not the problem. The transmitter was bad and was replaced under the 2-year warranty. (1-year for the watch)
The "wearlink" belt for the transmitter is awesome. Very comfortable compared to the hard plastic one I had before.
The accuracy is very good after you get to know the quirks and become good at the manual calibration. I never calibrate using the automatic "run" cal feature.
1) The foot pod fork must not be able to move for and aft. This does not require shoelaces to be extra tight, but rather the fork must be "stretched" between the laces for and aft.
2) I use a minimum 4-mile flat course for calibration check.
3) Different shoes can require unique calibration but I found much less difference between trainers once I discovered issue #1 and tightened up the pod. Lightweight flats do require a different cal but once you know these cals it's just a matter of entering the calibration factor manually (which takes only seconds)
Certainly it takes a bit of work to sort all this out at first. I found a spreadsheet made by one of the members here
http://health.groups.yahoo.com/group/S625x/
which allows me to correct runs for distance error and zero in on the correct calibration factor quickly.
On my 20 mile runs I am usually within .05 mile ( I ran a 20 mile training run marked within the Martian Marathon course and showed 19.991 at the end)
It has quirks, it has problems, but all things considered, for me it does what I need.