I did this trip last year - the first one - and it is an amazing journey. You don't do it for luxury (although you do get two weeks on a cruise liner), but it is, as you say, 'a hell of an adventure'. As well as visiting some remarkable places (I would put Lake Baikal, Mongolia, Beijing and Alaska as my favourites), you make some wonderful friends - I laughed for three months.
It is split into five totally different parts - two weeks across Europe to Moscow; the Trans-Siberian Railway with wonderful stops at Lake Baikal and Mongolia; China; the cruise liner from China to Alaska; and finally crossing North America, which is not just the normal big cities (although you do visit the likes of Chicago, Seattle and, of course, New York) but you get to the heart of some amazing wilderness - all of Alaska, Yellowstone, the Grand Tetons and the Badlands).
I would thoroughly recommend it - I'm now looking to do their London to Sydney trip as well.