I sent for my two $40 off certificates, took them to Best Buy, and came home with two of their converter boxes: rated better but not best by Consumer Reports.
It all seems to work fairly well, with one (geeky) caveat. Most of our existing (non-digital) stations transmit in the VHF band: you use the long telescoping antenna on top of your TV. All of our digital TV signals (except KCTS, channel 9) are UHF channels, which you receive with the little circular or bow-tie shaped antenna on top of your TV. UHF signals travel "line of sight", so you may have problems with set-top antennas and the Queen Anne transmitters (channels 4, 5 and 7). I find I can get some of those stations only upstairs in my house.
Digital TV is essentially "all or nothing": no weak, snowy picture - you either get a great picture or a blank screen (or a picture that freezes and breaks up into lots of little boxes). I've noted that planes flying over can cause that latter sort of interference. The digital bonus feature is that several of the stations actually broadcast multiple digital signals.
Conclusion: it works, it's easy and (if you don't have cable) you'll really have no choice after June 12th, 2009. You might need to spring for an external antenna for channels 4, 5 and 7.
[Update, 9/18/08] See comments for updates (from KCTS) about which stations will switch back to VHF after the transition. See this KING-TV article (and links) for much more info.