I bought a Mantis a couple of years ago and have never regretted it. It is not to be confused with a full blown rotavator which will plough through even the toughest of virgin ground, but it is excellent at quickly getting a half worked area into a fine tilth ready for planting.
The first few times I used it, it brought all the stones up to the surface and some of these did get stuck between the tines(?) and needed unblocking, but after a few passes it was a piece of cake.
I also bought the lawn scarifying attachment which may well be worth considering if you have lawn area.