Thanks again for all those who replied. I decided to start with the classic crime collection, and am reasonably happy with it.
For the benefit of any others who might take the same route, here are my thoughts:
- The collection contains a good mixture of first-class writing and absolute rubbish. In the first category, I'd include The Woman in White and Riddle of the Sands. In the latter, Rex Stout's awful potboiler Under the Andes (I'll bet he'd have hated that work seeing the light of day again). There's also plenty of Agatha Christie, Arthur Conan Doyle, Wilkie Collins, Edgar Allan Poe, and lots more.
- As far as I've seen so far, the formatting is good. There is an active ToC for the whole collection, and sub-ToCs for many of the individual works.
- I like having over 100 separate works all under one entry in the Kindle Home menu, rather than cluttering up the menu with so many individual items.
- Biggest problem: The collection includes short stories, novellas and full-length novels, but unless you are familiar with a particular title, it's hard to know which category it falls into. I don't like embarking on a book without any idea of whether I'm going to be finished reading it in a few minutes or if it will occupy me all week. You can get a rough idea by seeing if there is an internal ToC, but that doesn't really help that much.
On balance, I like the idea of buying these big collections, and will probably go for some of the others in due course.