While there is no arguing having another person to drill with is the best, I still use my grappling dummy ( I bought a submission master dummy a few years ago, and its still holding up pretty well), mostly for top game. You can do most of the open guard passing warm ups on one pretty easily without having to reset the legs since they are in a permanent hip flexed position. Also just going through the movements of the berimbolo is pretty good too, just getting used to inverting with controling that foot.
However, without a doubt, if you have access to anybody you can drill with on a consistent basis, I would suggest saving your money. Just my 2 cents, tho.