There are a lot of reviews on different sites. The reviews are generally mixed. It seems that if you have major trimming to do, then this product may not be the best option.
Some reviews:
http://www.reviewcentre.com/reviews98240.html
"I'm extremely disappointed with this garden groom unit. As others have stated, if you have more than about 1 inch growth, the unit housing merely pushes the growth away from the cutting blades."
"The Garden Groom is the most useless piece of junk i've ever bought."
"I fell in love with it."
"Providing your hedge has nothing thicker than 3mm the Garden Groom is OK."
"The Garden Groom is the best tool I've bought in a while!"
"The Garden Groom Hedge Trimmer is a great idea in theory, but not robust enough in the real world!"