Personally I wouldn't waste the money. Seems so small and dinky, too much time spent picking up the few balls it can hold, plus like you said 15 mph doesn't really emulate real tennis at all.
You'd be worlds better off if you could just volley back and forth wit a real person every couple days. Heck I kinda feel like trying to volley off a wall might be more productive than using that machine.
If I ever bought a ball machine I would want to set aside enough to make the purchase not only productive, but useful for more than just one thing.
Just my personal opinion, take it with a grain of salt.