I usually buy tickets the evening before, to avoid queueing on the day. I used to buy at the TVM, but since GTR Southern have replaced their machines with new ones that do not allow selecting a different origin station, that is no longer an option. Buying online is too fiddly on a phone for a simple walkup ticket - entering address and credit card details and being forced to choose an itinerary takes ages, especially around London Bridge station where data signal is virtually non-existent. As such I now almost always buy at the (Southeastern-run) ticket office.
So, for my purchasing habits, these new TVMs are helping keep ticket offices open!