Accessories

Better than the machine itself
I couldn't find one of these at any of the "big box stores" so I got it from here, which is better anyway since I probably would have paid more at the store I'm sure. These wheels don't wear down for a long time. I've still got the original in my 500X and that's after sharpening at least 100 bits many times over. In my opinion the grinding wheel is better than the machine itself, at least the 500X (see my review for that if you wish).

Have a spare one around
If you are lucky enough to own a Drill Doctor, which is by far the best drill sharpening tool I have used to date (short of industrial machines costing 10x more) then you'd best grab a couple of these replacement wheels. While the diamond wheels will last many hundreds of sharpenings, perhaps even more, they will eventually need to be replaced when the diamond wares enough to slow the sharpening operation.

Spare wheel
I originally bought this as a spare sharpening wheel for my new 750 Drill Doctor. I sharpened every drill I could find (about 150) and I still haven't had to use the spare yet.