This is designed to stop your chain falling off the smallest sprocket. On any bike with a double or triple, the front mech does that pretty well already.
Where the ChainWatcher scores high is for 1x9 or 1x8 set-ups where there's a single ring up front - a hack bike or a kid's bike, perhaps. The split plastic bracket tightens on the seat-tube with plumber's metal hose clamp (included). It's neater and more effective than a'sleeping'front mech on single-chainring bikes, though a bit pricey for what it is. Solid plastic would be stronger too.