It really depends on what you are after, for macro work, I think bellows make the most sense. I bought 8mm and 16mm extension tubes to decrease the minimum focusing distance to create tighter portraits. Stacked, these really allow for some close focusing!

Go to the Accessories (bottom right column) for info on tubes, bellows: http://www.hasselbladhistorical.eu/HW/HWVSys.aspx

And access the calculator here for exposure compensation: http://www.hasselbladhistorical.eu/HT/HTCuC.aspx

You can also search flickr.com for hasselblad and bellows or 8mm or 16mm (or higher) and you will get some nice examples of what the setups can do.

PS: Since I don't have an "E" body, my extension tubes aren't the E versions - I found my pair pretty cheaply because they were the older style but were still in great condition.