I have to say that apart from Ten, Captain Jack is my favorite. And yes I agree that in Torchwood he is more serious but it's also because he is in charge of his own team as opposed to following the Doctor around. I don't think that it would be a good idea to bring him back for 5, just because he had such good chemistry with both Eccleston and Tennant, it would ruing to meet another Doctor.
Agreed! I LOVE Jack but I'll prefer him NOT to be with another Doctor... Torchwood should be enough for him... He's fantastic!