Montag has lived most of his life with only the illusion of happiness so I think that Bradbury wants to reader to understand that in letting go of those illusions that life will certainly be difficult, but it will also be rewarding. Montag will know real disappointment, he will know real pain, grief, anger, and all the emotions in the scope of the human experience. He will not live his life among illusions, but he will also get to fully experience happiness and joy all the while figuring out who he wants to become rather than who society forms him to be. He will have the power of real applicable knowledge and not simply facts, he will be allowed to slow down and fully taste all the wonders the world has to offer him. It will not always be happy and easy, but the joyful moments will be much more appreciated because they will be real and in real contrast to the actual moments of pain which makes them even greater.
Bradbury does not make the life of the book people sound easy. First, we know that they are constantly changing their location, so not to get caught. Also, although they keep themselves clean, it is a difficult task: they maintain long beards and their faces are constantly burnt by the fire. Most importantly, they have all left their families. Despite these hardships however, I think we are supposed to assume Montag will be happy with the book people because it is more satisfying. He can finally enjoy literature and knowledge with others who also enjoy it.
Bradbury has Montag join the book people because Montag becomes one of them, memorizing the Book of Ecclesiastes. Montag is welcomed warmly into the group and made to feel a part of it even when he tells Granger that he doesn't belong because he's "...been an idiot all the way." Granger then asks Montag if he wants to join them and Montag answers simply, "Yes." After the city is bombed, Montag walks toward the city and the others fall in behind him. This shows that the book people trust him and his judgment, a feeling which is mutual.