First thing first - Ghajini is not a scene-by-scene remake of Memento, though it most definitely is 'inspired' from it.
Surya has 'short-term amnesia', which basically means here that he can remember events only for a duration of 15 minutes, before the next bout of amneisa hits him. The puts a spanner in his plans to kill everybody who conspired to kill his betrothed (Asin, who is the life and soul of the movie. More about that later). The whole movie is about how Surya, aided and abetted by a hugely overweight Nayantara, succeeds in this.
The pluses: Surya, Asin and the music. Surya has come a long way from his 'Nerukku Ner' days. He emotes well (though I thought he overdid his amnesiac portions), and shares a great chemistry with Asin. Asin is a revelation here. She handled the funny bits as well as the unique bits of her character with aplomb - an actress to watch out for. A couple of the songs, especially 'Oru Naal', are catchy and hummable. The action sequence in the climax was very well executed.
The minuses: Nayantara, the villain(s), the song picturizations and a shoddy screenplay. Nayantara looks like a dumb bimbo, and dowright vulgar in the one song that she has. I even thought that she looked like Mumtaz in a couple of scenes. The villain was pathetic, and I didn't even figure out why the twins bit was roped in. Most of the songs were picturized poorly; the director and/or choreographer seemed to have run out of ideas. Loops in the screenplay stood large, as did certain stupid parts - could have been evoided.
Verdict: worth a watch for Asin and Surya, in that order.
Update: HawkEye's review says pretty much the same, but in much stronger words :). Check it out here.