I've had a Tennis Tutor for over ten years and find it very reliable. In a decade the only maintenance has been a single battery change that cost $75. It's fully portable (but kind of heavy) and you can vary ball pace, pitch, and feed rate. It also has a plate on the bottom that allows you to set the machine to rotate and spray balls that you have to chase. It was $1,000, but I figure with the use I've given it, it's cost me maybe $2 -- $3 a session.