I used Ortleib front and rear bags on a long tour through Australia. It didn't rain much so they weren't tested thoroughly from that point of view, but being watertight and also airtight does mean that they act as useful floats should you drop your bike in the river, which is what happened to my friend on the same trip! Apart from that, they worked fine as things to keep your kit in, but the clips at the bottom of the bags were hopeless at staying on the racks (can't remember what make the racks were), so we ended up having to use bungies and wire to keep them in place as we bounced along the rough tracks. Even on a bit of rippled tarmac they came unclipped. The bags were okay, but it was a bad bag/rack combination. Also, think about whether or not you want pockets, which you can't have on a fully waterproof bag. You can see some photos of the bags in Australia at
www.red-line.moonfruit.com.