What monad would you use them for? I've never seen a really good use case.

I think LINQ is a special case of a monad comprehension. Syntax checked, type checked, composeable query languages (of which the "Structured" Query Language is only one example) are good things.

So far everywhere where I might have been tempted to use a Maybe monad comprehension, applicative functors were more clear anyhow.

You are right about query languages being a good for for monad comprehensions. See Torston Grust's dissertation[1], specifically "Combinators from Monad Comprehensions."

1. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.88.9...

Probably the Maybe monad most often, in usual code.

Or really as a shorter way of writing any smaller monadic code blocks.

I really look forward to using this new feature.

I don't know for sure until I try, but I think can be useful for logic monads.