I never owned the Super Gameboy either. However, I too had a friend who had it. Ironically. I had a bunch of Gameboy games, and he didn't even own a Gameboy, let alone any Gameboy games. He bought the Super Gameboy thinking that the games were pre-installed into the cartrdige. Obviously, they aren't. You are supposed to insert your Gameboy cartridges into the Super Gameboy. But he didn't know that. I tried to buy it off of him, since he didn't even own any Gameboy games. But he wanted to keep it, since he spent so much on it. Also, he wanted to borrow some of my games so that he can try them out for himself, since he never played any Gameboy games.
In the end, he ended up keeping the Super Gameboy, tho he decided not to buy any games for it, and decided that he would just keep it as a collector's item. I continued to try and buy it off of him, but he woudln't go for it. So no, I never got one for myself. However, I did get to try it out a couple of times. It's exactly what you think it will be like. Exactly. Still, I'd like to get one myself one day. I keep forgetting about it. But one day... one day.