I have one. I got it a year ago at Steve's annual convention in Columbus. If you can coordinate your tri-annual travel through Columbus with his May convention this year, go for it. It's not a disappointing event.
The 721 is not as popular as the 621 because the 721 isn't the "first year". My understanding is it's essentially the same TV, except the 621 has a smaller and unobtanium 7DP4 crt, the 721 has an easier-to-find 10BP4. The 621's cabinet was designed before the war, so it has an "essence" of a pre-war set. That makes it popular too.
I wouldn't say the 721 is less popular than the 630 though. I think there are a lot less 721 survivors than 630s. I think the two have about equal bidding appeal and prices on eBay.
Mine doesn't work yet. The CRT and tubes check good and everything's there. I'm just saving it (and my Zenith color roundie) for a time when I'm more experienced at restoring. Doing less valuable & less difficult ones first until I'm confident. I expect it to work fine later.
