First of all, Talaash is not Kahaani and so people please stop comparing! Talaash is a well made mystery thriller with a frivolously handled climax. However the way the movie unfolds gives you a hint of something mysterious to happen. As a matter of fact, the starting song sequence itself had a spooky mysterious feel. The director is playing in between real and surreal and it is never easy to make such movies as you have to convince people about something they don't believe in! But still the director has done a brilliant job to make people keenly interested with the gripping pace and story. The second half is a bit dull but only relatively. In fact many important sequences unfold in second half. Different aspects of human behavior are traversed and handled proficiently.
All in all, Talaash works. It's just that the climax is frivolously/amateurishly handled but still I will give it to the director for daring to make such a movie in bollywood where everyone else is playing with same old romance/comedy/telugu action genre. Hopefully the next one will be far better by her! For this one, please be more receptive and don't watch this movie with your presumptions! 7/10 from my side.