Don't worry, this one didn't come from the sidewalk! While we were at the grocery store the other day I saw a basket of mangosteens and thought for $1.98/ea we'd give them a try!
The outside is a lot like a pomegranate but there's a thick flesh between the outside and the fruit inside (almost like the white stuff on really fresh grapefruit). It was actually a little tricky to cut it so that I got the outside off but didn't cut into the fruit.
The fruit itself is slippery and is kind of like a peeled grape. It tastes kind of... like eating a Johnny Pop-up. It has a slightly sweet fruit-ish taste but when I ate one this morning I couldn't help but think it tasted sort of like eating pop-ups. I think it would make a nice addition to a fruit salad though - a lot easier than trying to take the membrane off oranges (for reference the fruit itself is about the size of a Christmas orange - maybe a touch smaller).
So that's another one off the imaginary list of weird fruit to try!