I'd heard of a rambutan before, and I even knew what it looked like, though I'd never had one previously. I bought these from a street vendor in Puerto Morales, a small village on the beach in Mexico's Yucatan. His sign said muy dulce (very sweet).
He showed me right at his cart how to gently score the exterior with a sharp knife and the insides will literally pop out.
The texture is very similar to a peeled grape. The flavor was more like a lychee - it's mild and sweet with just a hint of citrusy tang.
There is a single pit inside, which comes off easily when the fruit is really ripe (or so I read online; it also sounds like there might be different varieties, and some are 'freestone' while others are not.) The thing I found most interesting in my online research is that these do not ripen after they are picked. They must be picked ripe and then eaten, otherwise they will simply rot, but not get any more ripe. That is probably why we don't find them too often in Seattle! I didn't come across many uses of them besides eating fresh. They were tasty and refreshing and unusual; not amazingly delicious, but completely palatable. Definitely grab a handful to try if you have the opportunity!