I prefer watch-em-do-its. I want to know exactly what the evil character is doing, right from the get-go. The suspense comes in as the reader wonders if the victim will catch on in time to thwart the evil-doer. In this genre, there's nothing to figure out. The reader knows everything, going in. We hover over the scenes and watch the evil deed unfold. Now that's interesting.
The only way a who-done-it could be interesting for me is if it was a real-life story told in a who-done-it style -- in other words, a true crime novel. That it's real, makes it interesting. I've read my fair share of true crime novels. In the able hands of Capote, or even a contemporary like Ann Rule, these tales can be gripping.
In a who-done-it, you're only trying to figure out where the writer hid the Twinkie. How is this a challenge? It's arbitrary and meaningless. (I have the same reaction to crossword puzzles. I've never understood their attraction. It's about the talent of the clue-provider as much as the puzzle-doer's perspicuity. And really, what are you accomplishing? Now, if the aim was to figure out a new, real-life Rosetta Stone, I would find this interesting. But some guy's puzzle? Nope.)
A who-done-it is just a shell game. What do I care where the novelist hid the salami? It's a boring exercise because it doesn't matter. You haven't figured out anything except what some guy thought late one night in his darkened living room. I don't get the attraction of this genre at all. In real-life stories, when you figure something out, it means something. Who-done-its? Nonsense. There's no there there.
But, you say, "There's the writing to consider! How could you not even speak of the writing! You cad, you!"
This only reminds me of religious apologists who insist that atheists read the more "sophisticated" theological texts. Get bent. These "sophisticated theological texts" are philosophical musings about nothing. Again, there's no there there. So no, the writer's talent doesn't make reading a who-done-it meaningful. It's a who-done-it, which means it's nonsense, no matter how pretty the words are.
(I fully expect several million commenters to disagree with me on this. I'll probably get 160 or so comments in the thread. Dang. I'll be deluged!)
