I don't yet, but I'm thinking about it. I don't feel like I need it for cadence... I do cadence on my trainer by pedaling in time with music whose tempo I know, and I've got my brain calibrated well enough now that I can figure out pretty reliably what my cadence is on the road just by feel. I haven't felt like I needed a computer for mileage in the past because I know the mileage of most of the rides I do regularly, and I can pretty easily work out the approximate mileage of new rides. And I don't have a HRM, though I've thought a bit about getting one.
I would >love< to have a computer that monitors wattage, since that's a great parameter to keep track of when you're riding, but that requires a power meter, which I don't have.
But put all of that aside. The Madone I'm waiting for is all about speed, and I just know that once I've got it, I'm going to want to know how fast I'm barrelling down the road. So if anything's going to drive me to get a computer, that'll be it.
One question for those who have 'em... One of the downers of advancing age is that my close-up vision ain't nearly what it used to be. A number of computers that I've seen have displays that frankly look like they'd be close to impossible for me to read without my reading glasses. Anybody got a favorite computer that's got a really easy-to-read display?