I've been with Explore three times and Journey Latin America once on trips - not just trekking. In my experience, they're well organised and offer the advantage of being able to go to some great places without having to do much planning for yourself. And good value. The size of group has varied between 5 and 14 with a reasonable mix between couples/singles. Maybe I've been lucky but I haven't encountered any problems with the group dynamics and met some amazing people along the way. Much of that is down to the individual, what they want and their expectations (not least of what they expect and want from the tour leaders - some people can be quite needy in that respect). I've always found that even though you're with the same group of strangers for a couple of weeks or so, you can find your own space if you want to. As for the tour leaders, three out of four were excellent - the third was a local tour leader in Thailand who enjoyed Mekong whiskey a tad too much. Edited to say that I think average age with Explore may have got older since I first used them.