They taught me to think for myself.
Yes, you read that right, and no I'm not being sarcastic.
If I had walked into the church down the street, I might not ever have walked out. I'm sure the doctrines are easier to swallow, and the freedoms are greater. But because of my time with JW's, I learned not to place blind trust into any organization.
Because I gave my trust to an organization that did not deserve it, I learned to place my full trust in Christ, himself. I now look at teachings, doctrines, and the men who teach them with a more discerning eye... and I examine my personal beliefs to be sure that I am staying true to my conscience and the love that Christ showed us.
I can't speak for anyone else, and I might not feel the same way had I stayed in my whole life, but I can only relate to you my personal experience.
(Please don't yell at me. I just feel it is dishonest of me not to admit that I think some good came out of it... at least for me)