I heard that, once asked about which guitar model was his favourite, Keith Richards said "give me five minutes and I'll make 'em all sound the same." [1]
The same theory applies for pro PC gamers, an unusually high amount use $10 microsoft optical mice. Then there's the majority of casual players arguing over the details of the latest $90+ mice with bells and whistles.
microsoft-branded optical mice tend to be of very high quality; besides my (steel) keyboard, it is the only peripheral that I have used for more than 10 years.
I use a $60 logitech mouse for everyday use. I love the sensitivity (5000 dpi laser mouse) and find it awkward do use other peoples mice. Of course, I'm no pro gamer, so your statement is probably backed up by this.
I used to play WoW at a relatively high level (no server firsts or anything, but end game raiding). I didn't always have the "best" UI add-ons installed and did some things that were considered beginner mistakes (keyboard turning), but I knew what I was doing and I knew the game well so I still performed well.
I actually had a good reason for keyboard turning, for those familiar with the game. I was a healer and used Healbot to let me click on player names to cast healing spells. Thus, I preferred to have my mouse free for this and used the keyboard for turning, which was minimal.