I haven't personally used any of the knockoff batteries but I have looked into them before, at least for the 18v line. The one common complaint is that you most likely won't get the actual capacity that they claim. Just skimming some of the comments on a few of the 40v batteries on Amazon, it seems to be a similar case.

This battery was the only brand I had actually heard of before (I think I bought a cheap charger of theirs in the past). According to one of the comments the 6ah battery actually weighs less than a Ryobi 4ah battery by about 20%. They claim to use the same LG batteries as Ryobi, so that seems a bit suspect to me.

Long story short, yeah you'll save some money but you likely won't get the power or run time you'd expect from a 6ah battery. I would either hold off until you can afford the Ryobi brand batteries, or split the difference and get a Ryobi 4ah for ~$110.