I have a 550ex already. I was considering buying the transmitter and a couple of 420s. Fortunately I had some good advice from several of my firends. The 550EX can do the same job as the transmitter. So, if you already have a 550, why not just spend the money on an extra 420EX instead? You get one more flash, and the 550EX can function as your master. Duh.
It was also impossible to find in any store in Seattle, (yes, I tried Glazer's.)