This was actually a gift for my cousin--Vitamix had an accessory sale, so I offered her a choice (she has a 750).
Mind you, I thought it was kind of silly but she LOVES it. I induced her to watch the Blending with Henry review of the product on YouTube so she'd avoid mishaps, which worked out really well. I should also add that I bought this same accessory for another cousin's son and DIL, along with a 5200--my favorite Vitamix--to replace a fading personal-size blender that starts with an N. They love it as well.

Bottom line: the only people who seem to have a problem with this are those who don't seat the container properly on the base. Don't hesitate if blending directly into a personal cup is important. This is a quality accessory; I have similar cups for my Ascent 3500 and they do a great job of keeping your drink cold.