As already said, Ghajini is not another Memento. They are like north and south pole meeting through some common longitudes like short term memory, the passion of main lead character, methods of remembering things etc. But the story telling approach is totally different. So the must condition to see Ghajini is to forget Memento before entering the cinema otherwise you would end up wasting your time in comparing these two instead of enjoying some wonderful acting, excellent songs/music and some heart warming scenes.
There is no point in telling about Aamir, the perfectionist. As always, he has done true justice to the role of a man suffering from short term memory syndrome. He is a bit loud in expressions in some action/intense scenes but that is his character's nature and also demand of the story. Without this, you would not be able to believe some of his fights where he is fighting men who look stronger than him. He is passionate. He has only one goal in life. If he hits, he hits with all his power.
The surprise element is the leading lady, Asin. She is a kind a character that will bring a smile on your face when she is on screen. She is a part of few of the light moments in the movie where you will smile, some will also laugh. But there are few scenes, specially in serious scenes where she is kind of ok. But overall she has a done a wonderful job.
Now the time for bad guys. Well Pradeep Rawat is good but he could have done it better. Jia's character is important and interesting.
Overall, movie is good and a must watch. Don't go by other person's opinion. You never know, this may be the best movie this year for you.