Sound quality was excellent, caller ID worked, and the voicemail system was up and running right away; if you've got messages, a blue light flashes on the router itself. No status symbol appeared on the handsets themselves to indicate that a voicemail was waiting, though, which kind of sucks considering that your router isn't likely to be positioned somewhere you can readily see it; the only saving grace is that you get the stuttering dialtone when you pick up the line.
Overall, for $10 on top of your bill, this seems like a total no-brainer for any T-Mobile customer with a landline, especially since you can port your number -- and the VTech handset isn't a bad little cordless, either.
Update: Folks have been pointing out that the voicemail button on the phone should light up when you have messages, but for us, it wasn't -- we checked it thrice. Just a word of warning!