Now, I saw this movie with five girls who thought it was great, so your mileage may vary....but I thought it was just so-so. The big problem I had was with the two main characters, both of which I didn't think were likeable. I mean, I wasn't rooting for them at the end because they had both turned me off so much.
As for Robin Williams, I like him, but he didn't have much to do in this movie. He played the minister, who was really pretty creepy (bugging their apartment, having a mini-me of about 12 years old). If you missed it in theaters, I wouldn't even bother giving it a whirl on DVD.