I have used Second Life before and have mixed emotions about it. I love the ability to explore the ocean, African wildlife and creating weather disasters. However, I would never let my students use it because there is a lot of inappropriate material. Adult suggestive themes, materialistic ideals and scams are not what I want to expose my students to. I would show them the educational material previously mentioned but would have to pull it up out of their eyesight to make sure they are not exposed to adult themes.
I do like virtual worlds, but mainly those where students can be active learners. One I have used in my class that students seem to enjoy is Whyville. It can be found at www.whyville.net/smmk/nice.
Whyville allows students to explore a kids world with science, social studies and other educational topics. Some of my favorites are when students have to land a hot air balloon in a designated area using force, wind and other variables. Another cool part is when students sort different recyclable materials.
Virtual worlds can be a tremendous growth opportunity for students. Please use them with caution.