I've found, in my case at least, the belt is mainly decoration as the kilt will stay up quite well without. I do wear a belt on the occasions that I use sporran hangers in place of a strap. Usually a 1.5 or 1.75 inch plain leather with plain buckle.
I do have a 2.25 inch kilt belt, black, with a fairly plain chrome buckle for more formal wear. I think it does add something to the look but I wouldn't use it for everyday wear.
Matt's boxpleated kilts are normally made with no belt loops and a belt isn't worn with a vest so I go without quite a bit. Makes accessorizing that much easier.

I agree with the recomendation of the belts from Stillwater, very good for the price.

Mike