Some collectors like them and having standard, low shanks and vertical needles, they are strong sewing machines. But for me, I wouldn't buy a 306 - I would not even want anyone to give me one.
They take a 206 needle instead of the standard 15x1 - and if I remember correctly, the 206 needle is only available in two sizes of sharp, 12 & 14.
I have a treadled 316G which is very similar to the 306, except that it takes standard needles and a lot of the problems from the 306 have been eliminated. It's still loud and clunky and inelegant, especially compared to a 15 or a 201 or a 301. The 306 is even louder and clunkier.
The 206/306/319 machines didn't sell well in their time and they're not often sought-after by modern collectors or quilters, either. I don't want to hurt anyone's feelings - I just want to give you another side of the story.
There are good reasons for these machines lack of popularity over a 60-year time span. ;)