Has anyone been on an organised overland trip ie one with a company on a truck type vehicle with a company such as exodus or encounter? can anyone tell me if they are any good? I am looking at going on one for six months, travelling from caracas to quito, via tierra del fuego.

Hi
I looked at these a few years ago, and decided they were far too expensive. Study the itinery to see what is covered by the fee - ie, accommodation (not always), food (i think you have to add to a kitty additioanlly once a week) and admission tickets to certain sights. In fact, i found the only things guaranteed paid for were the travel and Guides.
Plus remember you'll be stick with 15 or so other travellers for a few months, I didn't feel comfortable with that prospect (but then I'm just unsociable!).
I would seriously look at costing this idependantly.