Paul,  I don't use the RS300x sd but I use the Polar S625x but still uses the same foot pod.  I have found it to be very accurate but you do need to calibrate it on a 400 meter track and run your standard comfortable pace for 3 laps.  When I used it at Rocky Raccoon it read 99.8 miles at the end of the 100 miler.  The only time I have found the distance off is during the Wasatch 100 miler where it read 112 miles but that run had over 26,000 ft of climbing so I think it threw it off.
In 3 years I have never changed the battery in the watch and I actually wear it all the time as my regular watch.  The foot pod battery is a standard AAA and will last through an entire 100 miler but I always keep a spare just in case.  The foot pod says not to submerge it but.... I have ran with it through mud and even did a river crossing at Western States and at Leadville with it and it is still working.

no real input on the nike thing, never heard of that.  I really have liked the polar products.  I still do also love my garmin 201 that I use for road speed drills with the virtual training partner it just does not have the battery life for ultras though.  
To give you the real "skinny" on my Polar 625X.  I bought it as a gift for my wife and then just kind of took it for year and used it until one day she started training for Ironman and then wanted her gift back.  Ironman is over and I stole it back again, at least for a little while.

That's what I was afraid of Ann.  I wouldn't mind sacrificing a bit of accuracy for simplicity and being able to use it for real long runs, but if it is wildly inaccurate I think I'm back to the Polar.