i've used london drugs a couple times and been very happy with the results - some as large as 16x20 too!
if you're not in a rush just watch their flyers for a sale and you can get some really good deals
i also found that they are really good if something goes goofy with the photo.
story time... the night before our daughter was born we ordered chinese food and my wife's fortune cookie said 'you will soon witness a miracle'. we took that fortune with us to the hospital and after jordan was born we put it between her fingers and i got this shot:
when we got them printed at london drugs we used the on screen software to crop the pic to a particular size and it looked fine. when the pics printed something happened and the pic was cropped so the fortune read, 'you will soon witness a m'. we showed the people there and the employee re-did the crop and reprinted the photos with the same result, so then she went and worked on a different computer in the back and got it to work for us. to me that was great customer service... even though they never did figure out what the software was doing on the computer we used earlier