I successfully bid with SeatBoost twice in December. The price increased from the first to the second bid. When I had the 3rd flight, the price was already above the upgrade at the airport feature and remained so in the following flights, so I just stopped using it altogether.
I noticed that. Their price is the same or more expensive than the upgrade directly with TAP or at check-in desk. And you only get the seat with them & meal, no associated business class mileage or baggage.