I just sold a novella I wrote, "Feral Moon". It's a military SF piece about the invasion of Phobos.
In some respects, novellas are the best form for science fiction, long enough to contain a complete plot or a crisp exploration of an idea, short enough to not waste your time on a bunch of useless crap. But there is one thing that is bad about novellas, and that's how much you get paid for them.
Or, as I put it to my friend Greg Feeley at Readercon, while we were discussing that topic: "Novellas: all the work of a novel, all the pay of a short story".
That pretty much covers it. I still like writing them, though.