mchack: Holy Crap, I didn't know Hidden Object games were that popular on steam: out of the 9 games 6 have above 90% positive reviews (two even 98%) and from the the other 3, two rate over 80 and only one has mixed reviews... never seen something like it in a bundle .. (though I am kinda new to bundles)
Maybe it's not that they are popular but people who dislikes these kind of games would not buy them, and, therefore could not review them on Steam.
Artifex Mundi games have more quality than most other creators of HO games. Some games in the bundle are older and not that good (9 clues is crappy, for example) but others like the Nightmares of the Deep series are among the best examples within the genre.
There's still the silly puzzles, strange stories, and all the cliches, but with better artwork, maybe better voice acting, and not buggy implementations. This bundle is a great entry point if you are curious about the genre.
Also, they have trading cards and achievements and the Steam crowd loves those.