I once used it early on in a deity game, when I was fighting very close borders with another civ. They were creating axemen who started with 3 free promotions thanks to deity free xp, and I just couldn't compete. Using that, I annihilated their biggest stack of doom, proceeded to take 3 cities off of them, and another 4 from two neighbors, and those 5 turns of not being attacked were all I needed to make swordsmen and archers in every city, and support an invasion army. Thanks to the economic advantage of them losing, and me gaining 3 cities, I eventually beat them with sheer numbers. It had many drawbacks, but march of the trees is one of the best worldspells there is. Do remember that the treants start with Woodsman II (double forest move), which makes it incredibly effective if your enemy has forests in their territory. It's like a free road network you can take advantage of, to outpace fleeing enemies. The New Forests that are left behind, also act as forests for that purpose, so you really have a range of 5-10 spaces, not just 5.