Back in the day it improved performance on older OSs, but with Vista/7 - and even XP SP3 - there seems to be overwhelming evidence of no improvements and in some cases losses.
I personally see improvements with it disabled in Windows 7 x64 on my HDD, so I disable it. Not huge mind you, but my primary reasons for disabling it now are to reduce the amount of data being written/read from the drive. I'm not at all concerned about gaining/losing 1-8GB of space.
I say do some simple timed tests of tasks you'll normally be doing and see if it helps and determine for your individual system. And if you're at all concerned about disk I/O load up the performance statistics tracker from task manager and compare the total numbers.
Regardless of what everyone else says, test it for yourself and see what happens.