I went to a childrens' home in Guatemala with i-to-i. I wanted the safety net of someone meeting me at the airport, taking me directly to my placement, and being available for any problems I might encounter.
I'm braver now, and wouldn't use them again, as I found out when I was there how easy it was to orgainise volunteer work yourself, and thus save yourself thousands. And once you're careful and do some research beforehand on where the best places are to stay etc, you don't need to be met at the airport etc.
Obviously it depends on where you're going, and how brave you feel. If you are going for a long term placement (more than 1 month), and want the security, then book the 1st few weeks through them, and then organise the rest when you're over there directly with whatever organisation you want to work with at no extra cost. i-to-i are flexible too, (or were last year when I was with them) so if you want to extend your placement through them when you're there, they'll happily arrange it.