I have routinely seen requests at clients with 2-3000 queries (even with query caching). Most of them are small, but at that amount it doesn't matter how small and efficient each query is.
I love AR to death as an OR library, but it is extremely easy to get into these kinds of issues when you iterate over a large dataset and then call associations of associations without thinking too much about it. I don't think it is only an AR issue, conceptually I think it is true for all ORMs.
They can be a PITA to unravel and are often very hard to do in a clean AR like way in more complex data models. Normally you end up doing some fairly un AR like pre loading like you would do in a pre ORM app, which while ugly works.
to those that don't know, Rails doesn't load the children of AR objects by default, so if you do something like
Select * from books
and then iterate through the books and get books.author_name where the author data is a relational table, you're going to get a separate query for each row.