He's faster than Gallas who played left back for a title winning team and he was better defensively than Cole, but worse offensively. You don't have to be the fastest player in the world to play fullback, its fairly essential to being able to be a fully fledged offensive fullback AND be able to get back to defend when required.
Clichy seems to want out, Gibbs isn't anywhere near ready to be first team, defensively he's very weak, physically he's very weak, offensively he's only had 2-3 great games, most below par. Clichy at the moment isn't playing particularly well, but Gibbs would be a pretty big step down, Vermaelen would be a HUGE step UP defensively for Arsenal at left back.
Personally I don't think Cahill is anywhere near good enough for a top 4 team, which is what we should be aiming at.
This transfer window is a joke, preseason training isn't far off, I'm fed up with Wenger waiting till August the 30th to buy completely the wrong players, again.