This is a great thread. For any portable device user.
Just wanted to add that I also bought the
airstash. With the airstash it's actually a wifi SD card reader. So rather than having fixed storage such as the widrive or go flex you can have many SD cards. Its a little pricey up front, it does come with an 8 gb card though. I bought a couple of 32gb cards. It works with the iPad just fine, there is an airstash app. For the kindle fire or other android devices there is a WebDAV lite app, according to the apps page on their site. I haven't tried it this way yet but I will this week & let you know how it works.
Take care, enjoy your content.
Here it is on amazon. $141 with the 8gb card. Would be nice if it was $100 without the card. But, it is what it is.
http://www.amazon.com/Maxell-AirStas.../dp/B006473T92