I have a 128 treadle and I have had two or three 128 hand crank machines which I refurbished and gave away. I still have two 27's, which are the larger version but basically the same machine.
They're wonderful for piecing! Lots of people even like to do their free-motion quilting on a treadle, but apparently I'm not coordinated enough to do that. :D
The stitch is beautiful on the 27/28/127/128 machines - but the bobbins don't hold much thread compared to a class 15 bobbin.
Finding authentic vintage bobbins and shuttles can be expensive. There are modern repro bobbins and shuttle springs available really cheap, but the repro bobbins are too short for the automatic bobbin winder to work properly. (That just means that you guide the thread with your fingers, rather than allowing the machine to do it.)
The modern shuttle springs work just fine - when you find that your shuttle tension is inconsistent (or non-existent) just replace the tension spring and you're back in business.
If the 128 that you're looking at is electric, you're probably going to have to replace all the electrical components or convert the machine to a hand crank. A new motor, cord and foot control is going to cost you about $50.
Hope that helps! :)