I jumped to all grain after a steeping kit and a partial mash kit, but primarily because a buddy had provided most of the equipment I needed to go there. That aside, I've been recently impressed at how excellent a friend's kit beers have been. It seems the main reason to take the plunge is when you want to work with Vienna, Munich, Rauch, Rye and straight wheat malts that will be hard to find as extracts. Otherwise, I'm quite convinced that most of the kit manufacturers have really dialed in those recipes. The rest is up the the brewer - good process=good beer.