They're all the rage, but I've not been a big fan.
I played one once - I think it was one of the Mystery Case Files games. The idea pulled me in - I'm an old adventure gamer, solving riddles and pixel-hunting is familiar! But after a few levels I felt sort of let down.
As far as I know, in all these games, the objects you're asked to find are random. They have nothing to do with the plot, if there is a plot. They're THEMED to go with the plot, but you're not actually finding CLUES. You don't DO anything with the objects you find. They don't MEAN anything.
(It's possible I'm wrong. Feel free to tell me.)
I remember long ago I had some sort of mystery jigsaw puzzles. You were supposed to read the story, assemble the jigsaw, and then stare at the picture until you saw the clues in the image that would allow you to solve the mystery. I also read lots of mystery case books when I was a little girl, one of which was themed around somebody having a 'photographic memory' and therefore would also show you a picture in which you'd have to find the clue that solved the mystery.
(Aha, now that I look it up - the books are probably the Cam Jansen series. Cam for camera, see, it's a joke?)
Wouldn't these hidden object games be more interesting if you had to figure out what the objects you found MEANT? If you really did solve a mystery with them?
I'd be more likely to play. :)
EDIT - The following games have been recommended to me as examples of hidden object games with more plot. I have not personally played them.
Agatha Christie - Death on the Nile
Mortimer Beckett and the Secret of Spooky Manor