Interesting shop, tucked on the second floor of an office building. I can't add anything to the review that Rhona-Mae A. wrote - her review is pretty amazing.
What I will say is that they have a good selection of music, and their online website is pretty awesome. I had actually called the shop for the book I wanted, as I found it on their website but prefered to go into the shop to pick it up. They didn't have the exact book in store but they had an alternative to what I was looking for. I came in and they had it waiting for me at the till - so nice! The person working there explained the differences between the book I was getting and the one I wanted and it just happened to be on sale!
Bottom line, they have really great staff who are knowledgeable and friendly, giving this place the feel of a family business. There is a TON of sheet music here, folks in the shop were just browsing through the rows. I expect their clientele is mostly built up of folks who frequent the place on a regular basis.