Well, a few weeks on and I can safely say I'm completely converted to these funny looking socks. Went out for a quick 3 miler wearing 'normal' socks a couple of nights ago and they just didn't feel right. I couldn't wait to get my Injinjis back on.
As for the shoe fitting advice, it was a total revelation. Very helpful. I came away with a new pair of the same old Cumulus that I've always been perfectly happy with, but as I'd expected, in a half size bigger than what I normally wear. I also took along my new Inov-8s, was told they were the right overall size length-wise but a bit too loose around the widest part of my foot, and was given some free insoles to put under the shoes' own insoles to pad them out a bit and make them fit better.
All in all, a very reassuring fitting session from a guy who obviously really knew what he was talking about.