I started a thread on this subject a little while ago.
On my heavy ground I have to say I found my Mantis it a little disappointing, but as Gold1locks has said they are probably better on light soil. Then again, on dry clay soil they do an excellent job in breaking down lumps of clay.
Also, I have found that unless the ground is absolutely weed-free, the blades have to be removed every few minutes to clean them.
I bought the more expensive one with a four-stroke Honda engine.