I'd guess about 99% are basically ok with reviews. The trouble is if only 5% of those bother reviewing and 1% of the bad ones post a bunch of fakes.

Incidentally I use argos.co.uk a bit and have found the reviews there always ok so far. Maybe Argos polices it or that crowd doesn't bother faking? It's different from Amazon in that there are no independent sellers so the only people motivated to fake would be the manufacturers.