While they aren't as romantic as the traditional treehouse, in almost all cases you're better off building a free-standingnstructure on posts. I grew up building many tree houses and am now in the construction biz, and when it came time for a "treehouse" for our girls inbuilt free standing. I am, however, still embarrassed to say that I built a kit instead of my own design. But coming home from work at night and seeing them out there in the swings makes me happy.
The kits are also much easier for those lacking that particular man skill.