It doesn't work, trust me. Diet cat food is full of carbohydrate fillers. The carbs turn to sugar and the weight stays on. If you aren't going to go to canned food, at least buy a high protein dry food. You don't want a diabetic cat someday!
Tips for transitioning to canned food:http://catinfo.org/#Transitioning_Dry_Food_Addicts_to_Canned_Food_
Protein/Fat/Carbs in common dry foods (you want higher protein, lower carb): http://binkyspage.tripod.com/dryfood.html