Plenty of people do well with the default dell mouse so like maynard said, this is CS, it doesn't matter.

I use a Steel Series Sensei because of how often I'm on AutoCad but it's all about macro handling in regards to that. However, when playing CS I can not tell the difference between the Sensei and an off brand laser mouse because it's easy to adjust the game/computer settings for the mouse to fit my needs.