I´ve been reading a bit about this camera, and some opinions go that it is too expensive a camera for a simple beginner, while others say it is an excellent entry-level one with enough features to go around for quite a while. So, should I just go for it or buy the Rebel XS or a cheaper one by Nikon? I have just the right budget to go for the 550D with the kit lens (it's only USD 600 on amazon.com), however, I definitely want to get the 50mm lens in the future, and also a good zoom lens. Do you think I should get a cheaper camera plus one of those lenses or just buy a "good" body as 550D is supposed to be and then play around and wait to get another lens?
The 550D is a very capable camera. The image quality is very good and the HD video is also excellent. It has quite a small lightweight body - it feels a little more toy like that the more expensive Canon Cameras. It has a packed feature set for the money and I'm sure you could get a lot of enjoyment from it as well as some great photographs (you can get great photographs from pretty much any camera). It has a large LCD screen on the back which is a very noticeable improvement when comparing it to some of the older Canon models from the same line (400D, 450D etc.). In my opinion it is not too expensive a camera for a beginner, it has a lot of features that you will enjoy learning to use.
On the downside it is a crop sensor (this may or may not matter to you and is the subject of much debate) and uses the EFS mount which means that if you later decide you wish to upgrade to a full frame sensor camera (EF) any EFS lenses you have purchased will be no use to you. You can however use EF lenses on an EFS camera but you need to be aware that the focal lengths will be affected. The low light performance on the 550D is not as good as on the more expensive models and this is probably the area that you would notice the biggest difference between the 550D and those cameras a little higher up in the range.
I would say that if you are looking for a decent camera for your hobby then the 550D is certainly a good choice. If you think you will be more getting serious about photography then it may be worth considering a full frame sensor camera from the outset as the lenses you buy will potentially last for years and years and if you get serious about photography then I can imagine you would start to lust for some of the features on the slightly more expensive models.
I believe that the most important thing is the lenses as these will outlast your camera body, so if you must decide on where best to spend your money then I would advise on better glass over a better body.
If you really are brand new to photography and the 550D fits your budget then I say go for it, you won't be disappointed.
I would agree that it is a good camera to start with, though to save some money you might want to go with the 500D, or if you can find it even the 450D? They would leave you the money to get the 50mm lens you wanted.
Between the REAL entry level EOS1100D and the xxxD series, I think that the 550D is not really an 'entry level' camera, but it will be more than usable for a beginner, and as you say, it certainly will have enough features to keep you busy for several years while you learn the craft of photography.
If you are looking to expand your knowledge in the future, I think the 1100D will frustrate you when you start to push its limits.