Here is my suggestion:
Instead of buying a ball machine, spend a smaller amount of money and rent one at a local club. Like most people, you will enjoy it for the first five times then get a little bored with it and move on.
Like a wall, a ball machine can be detrimental if the player is not using it correctly. Like the post of the little girl trying to learn to serve with the ball machine that FLA posted, she is developing the wrong grip, the wrong position, and certainly NOT learning to serve. (Maybe learning to hit overheads, perhaps!)
For most people who buy a machine, they forget that they also have to buy enough balls to fill it. If they use old balls, you are going to get inconsistent feeds and balls that don't bounce like new ones, of course. You have to load it in your car, find a court with a plug not being used, unload it, bring an extension cord, (unless you have one that is battery operated...which you better make sure the battery is charged and holds a charge!), load the balls, bring the basket of balls, set it up, test it, hit the shots, turn it off, pick up the balls, repeat, then load it back up in your car, take it home and store it. (Don't forget to charge the battery if you have one!)
I suggest you look for a good used one if indeed you feel you MUST buy one! Most private machines get very little use, for the reason I outlined above! Most people who have them would just like to unload it at half price or what ever.
I admire anyone who is willing to do all this to get use out of a machine, (and I'm not saying they are not useful...they can be!). I owned an academy and I had one of the most expensive machines at my club. It did get a lot of use. But it is rare to see someone who owns one out using it much after the first few times.
It even allows the coach to sit in a chair and get some sun!
