At first, I hated this movie. Amrish Puri was so evil he scared me to bits, and Madhuri Dixit's character (Gauri) seemed to be a nitwit. I have always loved seeing Madhuri in intelligent, strong and charming parts (Dil, Hum Aapke Hain Koun, Dil To Pagal Hai), so seeing her act like a 2-year old was annoying.
And then... Somewhere in the middle, the movie all started to "click", and I really started enjoying it. Shah Rukh Khan put in a fantastic performance which I absolutely loved. He was raw, yet charming; simple yet mysterious; and pulled off his role of deceived mute man who becomes 'enlightened' perfectly. A reviewer here said something about the fact that he could sing in the musical numbers but is supposed to be mute...but that reviewer obviously does not realise that songs are usually a suspension of reality in Bollywood movies. Characters usually can do anything and everything during a musical number! Besides, in most of the songs, it was a trick with Johnny Lever's character that 'enabled' Shah Rukh's character to sing.
Madhuri Dixit's character, Gauri, actually grew up and became much more likable. Madhuri ended up pulling off a fascinating portrayal of a young childish girl who is dragged into maturity by evil. Who wouldn't grow up, anyway, when faced with the terrors of Amrish Puri's "Rajasaheb"? And the relationship between Gauri and Shanker (SRK's character) is really sweet, complex and perfectly captured.
Rajasaheb, though terrifying, irritating, and very over-the-top, now strikes me as a fairly good characterisation. Like another reviewer said on here, a villain that we love to hate is hard to find. (see 'Dushman' starring Kajol, for Ashutosh Rana in a fantastic portrayal of another 'villain you love to hate' - much better than the one in 'Koyla', in my opinion).
I loved the other actors as well, the girl who played Rajasaheb's secretary was particularly good. Johnny Lever (though very silly as usual), the actor who played Lever's father, and Mohnish Behl (in a brief but strikingly tragic appearance)all put in well-rounded and credible performances. Very nice work.
The film is pretty violent and sometimes shocking (one particular attack actually made me physically jump), but I think it was a case of justifiable violence. The director, Rakesh Roshan, was trying to achieve a certain rawness, and he definitely succeeded. It does make you wonder, though, if he was going through some kind of "blood and gore phase" at that point in his film-making career.
All in all, 'Koyla' is very unusual for a Bollywood film, and that, in my opinion, is what makes it worth seeing.I started out hating it and finished it very happy. It is a satisfying and entertaining movie.