Been to the salt flats three times for Speedweeks. It's an incredible experience that everyone should have once. You'll see the strangest cars out there - cars running turbo'd straight six Chevy's out of garbage trucks, little four banger flatheads, V12 flatheads, big twin turbo diesels, everything. The racers are the nicest people too, they'll generally talk to anyone at any time. I even got to get down inside of one guy's bellytanker my first time out there. Don't know how those guys race staring through a series of mirrors.
But like LOT said - watch the sun. I was sunburned so bad last time I went that I blistered, not fun. It's intense sun that at times you don't notice. The spectating requires a pair of serious binoculars, but the real action is in the pits and hanging out with everyone at the starting line.