Sorry to add more confusion, but it kind of depends on your needs and personality, purpose, etc. (Lots of factors)
Now that I have one, I realize that I can do without all the options. The main reason is that although it is a good tool to practice for consistency, it will never, ever replace a good hitting partner. I really can't see myself on this machine for several hours, day after day. It gets boring.
I don't even use the oscillation function, after trying it out a few times. The reason is, I can see the machine turning and aiming side to side, and it is just too predictable. Rather, I just use it to practice consistency and strokes.
Also, although my Silent Partner is advertised to shoot out at 80+ mph, I find it useless to do so. (For one thing, it is hard to calibrate it to stay in the court, gotta use lots of topspin to keep it in)
My main purpose for the ball machine was to use it for teaching my two little girls (8 & 10), and it satisfies that purpose 100%. It feeds nice, perfect slow pitches at the same spot every time (given the balls are consistent). For me, I take a ride on it occasionally to gain some confidence, whacking it hard. The ball machine makes the average Joe look like a professional......
Don't waste too much on it, and don't confuse it with a real live good hitting partner. I think the reason that you see ball machines on Craiglist is because the owners had the expectation of it taking the place of a hitting partner, and it got boring after a few sessions, and just had these sitting in their garages.