I had it (imported from Spain, via Tesco's Brixton) when I lived in London. It was as you describe, sweet, refreshing, and crisp. My Spanish housemate told me what it was called. It was shortly thereafter thoroughly trumped by an elongated honeydew purchased from an Iranian market. That was by far the best melon I had/have ever eaten.
According to the web, this is the melon most often served in Spain. It is also called toad skin because of the mottled appearance of the outside. I'm pretty sure that my melon was ripe as it was very sweet and refreshing, sort of a less unctuous honeydew. The difference is that it is very crispy, with more body than watermelon. I wasn't too impressed with my 1st slice, but it's grown on me enought to have two slices for Sunday brunch.
I'd buy it again (until casabas come into season).