I visited London recently and this was on my list of yarn shops to visit. I originally wasn't that excited, having read some okay reviews, but I was pleasantly surprised! The staff are very friendly and helpful (and love to chatter), they sprinted up and down the stairs to check for stock, I got some of my yarn wound for free (would've had all of it wound if my dad wasn't falling asleep in the chair outside) and they have a great selection of quality yarns.
If I had to put my finger on something, it was that the books weren't organised very well and that some of the stock (esp. in Rowan) seemed low, but those were minor things - I can get Rowan at home, so it was mostly to check out colours. I can definitely recommend this shop to anyone looking for a cozy, warm, quality yarn shop!
I was very, very excited to go see this shop - maybe that's why I came away a bit disappointed. The staff was very friendly, but I found the shop a bit dark, crowded and not very inspiring.
I did like that they had some yarns otherwise hard to get for me, but their stock wasn't very impressive. The yarns I wanted to look at were hidden behind racks and mannequins, and what was in the cubbies didn't seem all that exciting. (Some of it also seemed a bit overpriced, but I'm not sure.)
Perhaps it was the fact that I came in around noon, and maybe their stock was low that day, but some of the things I'd been told to look for weren't there, and it just generally didn't seem deserving of all the hype it's getting. If I'm ever in London again, I'll probably pop by a second time, but it won't be the first stop on my list.
Bon Marché is fine if you're looking for your standard yarns, such as Rowan and Phildar. I also found it a bit overpriced, and the salespeople did not pay any attention to me (perhaps because I was a student that looked broke?). I wasn't impressed with their selection (lots of acrylics) but the store in itself is worth a look - it's huge and filled with brand and designer items.
I've visited La Droguerie a few times (once in Spring 2008, once the year before) and it's a cozy shop. I like how the yarn hangs in long skeins on the walls! IIRC, they only sell their own house brand, but it's very nice quality. I do wish they would give you a little tag with yardage and/or gauge, but I found the staff very helpful (compared to some other French stores) and willing to take the time to help you find what you're looking for.
They also have a cute (and huge!) selection of buttons, ribbons and other notions.. Definitely worth a visit if you're in Paris!