Avatar isn't bad. It's definitely one of the most visually impressive movies I've ever seen- though I wouldn't say it's amazing (overall). The first time you watch it, it's really enjoyable and you get really pulled into it. The second time you watch it though- you get really bored, and realize how bleh it really is. Each time you watch it, it just gets more and more old, and you realize how tacky/bad some of the acting and lines of dialogue are. I've only seen it like 4 times (twice all the way through), and I can honestly say that I don't have any desire to watch it again. Inception though... Love that movie. Another visually impressive movie- though it's not 3D.
Avatar is ultimately like Star Wars: The Phantom Menace. It's awesome when you watch it in a movie theater, but when you watch it at home, you realize that it kind of... Well... Sucks. Not horrible, but not really that great of a movie beyond the special effects.
Just skip Avatar and watch Inception if you haven't seen either (just kidding!)