Treadles are wonderful - they move at a speed easier to control than the electrics, and make very neat stitches. My treadles are a 1907 Singer 66-1 Lotus and a 1934 Singer 127 with Sphinx decals (pictured). I'm also looking for a handcrank - as it won't be attached to a table like the others, that will be my portable machine.
The Lotus and the Sphinx are both in tables. When they are not in use, they can be folded down inside the table, which can be used as a hall table or desk. The one below holds the Lotus. I don't have any good pictures of mine; McKenna has some here at her site.
Sewing on the treadles is just relaxing. I enjoy listening to the sound of the treadle, and the vibrating shuttle on the Sphinx (she kind of 'chatters' as she sews). The machines are extremely durable - the maintenance is 'oil where indicated' at the given time frame, rather than 'bring in to maintenance shop every six to 12 months.' There are 100+ year old treadles still being used (and one is mine!), which is evidence enough of the quality that is built into them.
As far as attachments - they don't use the newer ones (something about the design means the new ones don't work), so you won't have, say, a zipper attachment. There are plenty, though; I have a half-dozen attachments for my Lotus and I'm missing a few. Sewing on the Sphinx, I must admit I really haven't missed them. Even the zipper went on fine.
TreadleOn - the best one-stop reference for caring for your treadle, or restoring it.
The Treadle Lady - good information, and she does a wonderful job of conveying her joy in these machines.
Singer 66 - Basic operation of the treadle machine, just showing you the machine as she's treadling.
Antique Sewing Machine Display - She's sewing on one, and the video also shows several others on display. A wide variety of machines, one pre-Civil War. Very interesting!