I have not, my guess is the resale value would be 25-30% less than I have into it.
You can't go wrong either way.
If you have the space and the bank roll to go a little bigger in the beginning then I would say go BF20 but consider that the mill is just the first part of the investment. Tooling, accessories etc. add up quick. Make sure your budget considerations includes those costs and that will make the mill decision a little easier.