Like Mark above I've met people on the road and travelled with them for varying lengths of time. Sometimes just because we're going in the same direction, other times for security, now and again because we find we have something in common. I've travelled solo and I've travelled with other people that I knew beforehand and we've planned the trip and a couple of times I've joined 'organised' trips where I didn't know people.

But what I've never done is 'answer an advert' - i.e. set off on a trip from scratch - and travelled together - with someone who's essentially a stranger. Not because that circumstance has never come up but because I consider the chances of it going wrong to be too high. Travelling in the way it's most commonly talked about here is a pretty high stress activity (even if it doesn't feel like it directly) and the chances are more likely than not that you and the stranger will have a falling out at some point - you want to go east, he wants to go west, that sort of thing (and it will be a 'he' if you are). Quite how that difference will manifest itself with someone you've never met before is, in my opinion, too high a risk.

Travelling in parallel - where you go your own way and meet up every now and again - may be workable depending on circumstances but the chances of meeting someone in the first place who wants to do what you're doing but semi independently is going to be pretty low.