I compare Halo to games like Final Fantasy and MGS. Halo 3 in my opinion is the best FPS online to play in terms of what it consists of. For example, you can control vechicles, a variety of weapons in addition to the game types.
The series has now aged which is why MS continues to create new IP's such as Too Human, BioShock, Mass Effect, Gears etc. I wouldnt consider it overrated as much as something that has aged. You no when you buy a halo game what your getting, same can be said for MGS or Final Fantasy.
Would I buy Halo 4 if it came out? Maybe, just depends on what new features are added. I like the map editor and movie features in Halo 3 so as long as they keep coming up with new features I would not say its overrated.