I returned to tennis after a 15 year rest break.
I bought a Tennis Cube for $630. It's very basic, but it has proved to be just the ticket for me (intermediate player). It's adjustable for elevation, ball speed, and interval between shots. It will load a hopper of balls, about 65. It also has an oscillation feature.
The unit is compact and fits in the front seat of my Mustang. Weight is under 30 pounds so it's very portable. When done, I just plug it in to the charger and it's ready to go for the next day.
First I used Penn no-pressure balls. These worked fine (albeit stiffer than regular balls), but may have contributed to my getting tennis elbow (mostly caused by my one hand backhand), so I switched to Tretorn Micro-X balls. They play the same as standard pressurized balls, but do not lose their bounce. Very durable too.
I've used the Tennis Cube an average of two times a week for 6 months and certainly recommend it. It's shortcoming is that the elevation system is a bit imprecise, and requires some trial and error to make changes.
June 2012: Making a comeback after 15 years! Who's this guy Roger Federer?
PK Kinetic 7G, Bab 17g n-gut @ 50 & PK Q Tour 295, Wilson 17g n-gut @ 50