I agree with T1DM2012, once applied there is no needle left in you - it is a small flexible filament. You hardly notice you're wearing it at any time. Water proof up to a depth of 1 meter for 30mins with out any further protection - I know someone who scuba dived with one under wet suit for 45mins at depths of 10m and it still functioned after that, but wouldn't rely on that always being the case!
As for it's usefulness...............as the judges on Britain's Got Talent say..............a million per cent yes! Even if you just buy sensors for use now and again it's the difference between watching snooker on an old B&W potable TV and a 48" LCD HD Colour TV!