Depends on the film and what those extras are. Some of the recent releases have just had the digital copy but the rest of the content was almost identical to the cheaper, single disk release. Those really aren't worth it to me.
Now, Iron Man, Batman, probably Indy, Star Wars: The Clone Wars...I'll being buying the deluxe releases of those as they're almost a collectible to me. Smaller films, like Forgetting Sarah Marshall (which also comes out tomorrow), I'm not looking to spend a ton of money on extras for them as I just want the movie. I probably won't buy deluxe editions, if offered, of Hancock, Wanted or the Hellboy film either.
Then there are the deluxe, deluxe releases like the Iron Man and Indy sets with the Sideshow pieces in them or the Lord of the Rings sets with the Weta pieces. Those I rarely buy but will sometimes. I bought all three of the LoTR sets and I'm thinking about the Indy set. One of the coolest ones was the Da Vinci Code release with the working cryptex in it. Worst one I bought was the last Harry Potter film with the Death Eater mask. It looked good in the store so I bought it but it wasn't that nice once you got it open.