Jeez, it's expensive, though.
You get what you pay for - exchange rates don't help.
I'm currently researching options with respect to making my own gear, since there are a few things that I'd like that the Viscacha (or pretty much any other seatpack I've investigated) doesn't have.
How's it compare to the alpkit thingie?
No idea. I got fed up of waiting for Alpkit to get their act together and get their bag in stock so I pulled the trigger on a Viscacha instead. I don't regret it. I've pretty much given up on Alpkit now because their supply chain is utterly piss-poor.
Apparently andrewc has an Alpkit seatpack - obviously the stars aligned just so when he ordered it.
And how much stuff do you get in it?
Word of God (Eric Parsons, the guy who founded Revelate) is 8 litres expandable to 16-17 litres. Combining the seatpack and a backpack (Wingnut Enduro) I can carry enough clothing and the like for a 2-3 week off-road tour. That said, my touring setup has been fairly viciously optimised.