Depends on how big an area you have to do. I tried a mantis years ago, and unless the soil has already been broken up, that thing bounces around like crazy. I didn't really like the job it did either. Most of the time the weeds just wrapped around the blades. It didn't really chop them up and bury them in the soil like your front/rear tine tiller would do. I use a front tine tiller and just use 2 of the 4 tines if I have narrow spaces. Does a much better job.
If you want to try one out, check out a rental place. Sometimes they have them. Otherwise, see if they have a return policy in case you don't like it.