The idea of using those tyre sealant things always worries me to be honest - they conjure up an image of running a tyre which is riddled with punctures and which only stays inflated courtesy of the gunge inside, rather than the integrity of the rubber itself. Yes I know we all inspect our tyres rigorously before every ride, but...
Regarding the Stopn'go kits - I carry one of those myself but have never actually had to use it in anger yet. But - Top Tip though - I'd definitely recommend doing a dry run with it before you end up having to do a roadside repair. Last time I had my tyres changed I brought an old one back home to have a play; and although the kit is simple enough to use, it wasn't entirely obvious how it worked and I was extremely glad that I was practising in a nice warm garage rather than at a cold, wet, dark roadside!