I bought one of these as I wanted to improve my putting away from the course. Well its certainly done that. It means you can spend hours practicing 5 foot putts and experimenting with sets ups and strokes until you find the one that's most consistant. After a week on this I could putt 50x 5 footers in a row. Its 72" from the edge of the hole to the end of the putting surface so a five and a half foot putt starts six inches from the end.
So I thought I needed more length. The answer has been to buy two and using some carpet tape, stick them together to make a 11 foot putting surface with a hole at each end - put three balls to one hole, take them out the hole, and then putt to the other end. Practicing like this and additional sessions on real putting greens is such a cheap way to knock shots off your score.
Simple bit of kit, and with two you can do 11 foot putting practice.