I only buy cumin whole... stays fresh longer, more versatile because you can grind, crack, or leave whole as needed. I use enough cumin though that I have a grinder dedicated to it (a hand grinder, similar to a pepper grinder), which I mostly do a coarse grind with for things like fresh salsa, seasoning soups, popcorn, eggs, etc.
A lot of indian food (and indian spiced food) uses whole cumin seed... toasting it completely changes the flavour and is better in some dishes than untoasted, but it's very tricky to toast ground cumin.