I've used Zwift every winter since Beta, though I don't hit PRs on it.
In the last two weeks my 20 min power has jumped up 39 watts (albeit from a horrendously low baseline).
I find that racing on Zwift is completely unlike racing in real life in that a real race for me is way more stochastic, with way, way, way, way harder surges. A races in Zwift are much more of a sorta hard effort and then hold around sweetspot or threshold with minor hard surges, and then a sorta hard effort at the end. Namely due to the completely unrealistic draft. But that's perfect for winter time for me, so I try to do a few each week.
I like the mountain for doing threshold workouts, and I like the group workouts for similar work as well. In all, it's an excellent way to get in 45-60 minutes of threshold-type hard work that doesn't take so much out of you that you couldn't turn around and do it the next day and the next.
I've never had such effective winter workouts so far as hitting higher wattages as I have with Zwift.
My type of winter routine.