This Walgreens is a life saver! It's close to Planet Hollywood, Paris, Cosmopolitan and Aria and if you need water, snacks, toiletries, etc and don't want to take out a second mortgage to buy them at your hotel, this is your place!
I was there twice this most recent trip and even though it was crowded both times, the store was very well stocked and clean. Also, there were plenty of open cashiers so lines moved quickly.
This might be a little more pricey than a drugstore out in the burbs, but for being right in the heart of the strip, you really can't beat this place.