I've been using Okamoto 004's with one lady I see and we both can't tell that I'm wearing anything. The first time we used one, she actually checked to make sure I was wearing one.
I had a bad experience with Kimono's. I was using one with one lady I used to see and it broke. But wait, it gets better, it broke right when I came! I couldn't even tell until after I had pulled out. Thankfully, she was on birth control and we both got tested.
I later jokingly told her that the new advertising line should be, "So close to nothing, you won't feel it break!"