I always have cordless, I even use a cordless controller with my pc. Infact, it is the same cordless you find with xbox360/wii consoles.
There is next to no lag (if any), probably the cordless mentioned further up was from a bargain bin where quality isn't an issue. As for optical, revered joe probably doesn't use a real mouse mat designed for optical mice and tried using a book or a desk with a reflective surface. A joy about cordless too, you don't have yet another USB plugged into your machine and it doesn't get in the way when you try to move it (damn cables) and in my case, I sit some distance away from the computer so having no cables going between me and the computer is nothing other than a plus. Opposed to 10ft away, I am working 4ft away from the screen, possibly slightly over, which again is some distance, especially to have cables crawling.
Note to Caravel: Yes, use a mouse mat, a real one, not a plastic one made for balled mice. Optical are superior to ball in DPI, and for example Everglide do a great range of mouse mats and you can notice a real difference and that is between the basic cloth mouse mats to more expensive ones.
As for mice, if you want a decent one at a low cost, the microsoft price range is very good from experience. The one I am currently using is the wireless laser mouse 7000 which you can pick up for around £30 easy enough. Then you can go for better or worse depending on what you actually want. The mouse I am using comes with a rechargable battery and has a rechargable stand with it so you don't have to worry about batteries either, just put the mouse on the stand while not using it and ta-da, no hassle and no problems.
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