I've gone 100% VoIP (Vonage), and it works with TTY.
However, the quality can vary occasionally especially on a substandard Internet connection, so it's a bleeding edge move. It works much better with a high-quality 1-megabit connection than a very jerky/bursty 10-megabit connection -- latency stability is better. So get a videogaming-quality Internet connection.
I don't think I'd recommend the move for CapTel (yet) as CapTel is more demanding on a phone line than TTY. But when CapTel comes to Canada, I will test it with my VoIP and see if it's good enough.