It depends on the game and the extras included.
I've bought the GTA Collectors Edition since I'm a huge GTA fan.
I probably would have got the collectors edition of Halo 3 if I had a xbox 360 when it was released, since that Masterchief Helmet was really cool.
The Fable 2 collectors edition was absolute crap, especially since most people got the Halo clothes free anyways, (if you live in Canada, just download the language file) so all that was left was the Making of DVD which you can find over the net.
The X-Blades collectors edition is perfect example of extras to include, unfortunately I don't see myself playing the game for more than a week.
Any game that includes an action figure, or some sort of physical item that relates to the game(not including the Making of DVD's), I always highly consider getting.