I have used the yellow "BedShaper" for about 6 years and have saved a ton of labor & sore backs. It cost about $3700, hydrostatic drive, very fast. I use this on every new install job. I also use to slice good grass/soil and bury low voltage cable, electric dog fence wire, etc. About the only draw back is that it can not 'Man Handle' really hard clay soil as well. The salesman taught me a couple of tricks that help (put a weight on cutting wheel and use two pass's, one forward and the other back the other direction with a 3" gap between the two).
Anyway, if you install alot of new beds from scratch this thing is worth it!
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