I am currently on the Odyssey Overland London to Australia trip in Siem Reap in Cambodia and after 5 months and 20 countries I am still having a fantastic time and wish it could go on for a lot longer. Six months seems like a long time on a truck with 21 strangers, but it goes by very quickly and you might not get on with everybody all of the time, but you will make good friends. I have found Odyssey Overland to be a fantastic company to travel with and would recommend them to everyone, a very professional, knowledgeable and friendly company. I have even booked on the first Cape Town to Egypt expedition that Odyssey are doing in 2010! I can't speak highly enough of the care and attention that the whole group has received from the tour leaders Tim & Cheryl.
I am also currently on the London-Australia 2009 trip with Odyssey, I can only agree wholeheartedly with Robin, this trip has been absolutely amazing! The places you go to are brilliant, from huge cities to wilderness so far off the beaten track you won't have seen a Westerner for days (except the bods on the truck, of course). It was actually a bit of a disappointment when we started to return to touristy areas and heard English from pasty white people
I personally have no reservations about recommending Odyssey Overland, their service has been seriously above and beyond the call of duty, especially that of Tim and Cheryl, our wonderful crew. If you would like to read about our journey from the horse's (i.e. my) mouth, my blog is at http://partwayround.blogspot.com/ .
And word has it that the 2010 London-Oz trip is already fully booked! You lucky people