I picked one up to sit next to my frame and love it!! I often use different threads for top and bottom, so I just load up the sidewinder with one and the machine with another. Helps to not have to adjust my machine for bobbin, then back for sewing.
I loved it so much, I picked one up for my craft room upstairs. I use it to wind the bobbins for my Viking and my Featherweight. I was told by my Viking dealer that its not good to wind the bobbin through the needle (even though it was designed to be wound that way!!).
I did find that my first SideWinder doesn't fill the bobbin all the way, so I have to move the arm and press the green button to have it finish winding. Of course, then I have to be careful not to overfill