The BTS is a nicely implemented all-in-one solution that marries the convenience of cordless with the power of VoIP telephony, but it doesn't do anything that you can't do by plugging a cordless telephone of your choice into a VoIP router. However, it does simplify installation and reduce the chances of incorrect configuration for folks who don't get their kicks from futzing around with hooking multiple gizmos up and getting them to play nice together.
Another advantage is that its $149 price is in line, and maybe even a bit of a deal compared to what it would cost you for a two-handset 5.8GHz phone, plus a wired VoIP router. And it's definitely a better deal than rolling your own when you factor in the $50 mail-in rebate offer that's running as I write this. (You only get the rebate after 60 days of Vonage service.)
But when compared to other Vonage-enabled hardware like the Linksys RT31P2, the BTS has the disadvantage of not offering the option of a second phone line. So if you're thinking of using the BTS as a mini cordless PBX, you'll need to get another plan.
Finally, I tip my hat to both VTech and Vonage for the BTS' ability to leave WLANs unmolested. But jeez guys, this is a desirable feature, so why make it so hard for buyers to find out about it?