[mrstew]

I think you've gotten the wrong end of the stick with Mendeley. It's a reference manager with social features, some of which could be used (with work on the part of the user) for discovery / post-publication review.

PubUp, in contrast, looks like it is designed specifically for those two things.

Presumably to use PubUp I don't need to store my PDFs in Mendeley, to have signed up to their web component, to have found a group of people whose opinions I trust, yadda yadda.

On a tangent Mendeley also certainly hasn't sewn up the market: the majority of researchers still use one of a couple of older, more established desktop apps for their reference management. That's certainly not to say that it's not a good product, it is, or that they won't become an incumbent eventually, I reckon they probably will. It's just that scientists are a notoriously hard bunch of people to reach.