I've yet to visit IM but have been to Davenports three or four times when in London.
The first visit is always the most exciting and the novelty wore off after the second visit. The visits thereafter were really only because I had some time to kill and was in the area otherwise I wouldn't have bothered.
I suppose that I've been spoilt for a number of years by having Mike Danata's studio on my door step

I've been to House of Magic in Stourbridge a few times and John was very friendly although they don't carry much stock, and I had my fisrt visit to World Magic Shop this week and they were very friendly indeed.
I suppose that the morale is that wherever you happen to be, if you can get to a shop then it's a far superior expereince than shopping online - whichever shop you use.