To be exact in the extent of my knowledge, I've used Overdrive, which is a worse product than Gravity, both in the category of hardeners (according to the source of information I trust) in two grows, and both times I used them for longer than a week and at the end of flowering.
I recently spoke to the guy and he told me how Gravity is better and that the hardeners are used for just one week after you notice the buds stop swelling. I still have Overdrive, and he's recommended I just use it while I have it and swap to Gravity after it's out, and he's the guy that could sell me it...so yeah. To use his words, "You're in and you're out with it."
So no I haven't used it 'properly' yet, but I will be this grow, although I'm doing a lot different this grow than I have in the past, so it'd be tough to give you an honest speculation of how it performed, comparatively. But like I've said before in the posts, the guy who told me is a hydro owner who tests the products side-by-side and has grown for over 10 years. I trust him, and I don't see why he'd want me to use the product less (only one week) if it wasn't for the best interest of the plants.