Q: I have read that magic lantern firmware makes the camera better in movie recording.

That depends on what you mean by 'better.' It does add several tools and options that Canon did not include in their firmware by default which are very nice to have for video recording. A partial list of which includes: zebra stripes, onscreen audio meters, cropmark overlays, etc. A complete list of features for the 550D can be perused here...

Q: Does magic lantern firmware change dramatically the menus?

Magic lantern doesn't change anything about the regular camera firmware at all. Rather it adds itself on top of the standard Canon firmware and adds its own menu which is accessed differently (via the Erase button on the 550D)

Q: Will I have difficulty adapting to it if I'm new with this camera.

Only you can really answer this question. I will say, though, that Magic Lantern is firmly in the realm of 'beta' software... Or maybe even 'Alpha' software, depending on the release and the bugs introduced! While it is far more polished today than is was in early 2010 when I started using it, it is still pretty rough around the edges. If you're not comfortable with the fact that Magic Lantern probably won't always work exactly how you expect it to, and that it is an open source project with everyone volunteering (so sometimes months can go by with no forward progress), then Magic Lantern probably won't be for you... At least until it gets more polished.

Q: Will the manual be useless?

Nope. As the Magic Lantern firmware overlays the regular Canon firmware it doesn't remove anything that you would normally have access to in the use of your camera. It doesn't remove anything or alter the operation of any function of the camera, really, just adds new stuff on top...

Q: Also would installing this firmware make the guarantee invalid?

Technically speaking, yes, installing Magic Lantern will void your warranty. However since it is loaded on the fly and is stored on the CF card (it modifies nothing on your camera's system at all... As I said above it is best to think of it as something that is an 'overlay' to your camera's normal firmware), simply taking out the CF card and restarting your camera will return your camera to a state as though it had never had Magic Lantern on it. With this in mind, if you did have a warranty issue, essentially Canon would have no way of knowing whether or not you ever had Magic Lantern installed.