The current Publisher's Weekly has an interesting article, Romancing the Store, written by Debbie Macomber and a Harlequin editor. Basically a grip about how independent bookstores don't carry series romances. The gist is that series romances do a lot to shape the romance genre and to some extent all commercial fiction and that stores are missing out by not carrying them.
One interesting point that is made, which I'd never thought of is that series books are more experimental - editors are willing to take on daring/outlandish books because the investment is so small. And that the biggest trends in the romance genre grow out of these experiments (like romantic suspense and paranormals).
I don't read many series romances. As a relatively new romance reader, I feel like there are so many regular romances out there I haven't read, that I don't feel like I want to waste my time on series. I know I'm probably missing out on some really great stuff. But I hope that if editors at the big publishers are doing their job right (and we hope that they do at least most of the time) then really great authors/books will be pulled out of series and put into full-fledged romances.
This article also made me think of how sad I am when I go into independent bookstores and see that their romance sections overall are so tiny. Really, like one shelf. There are some great independents in NYC that I love going to--Coliseum on 42nd, McNally Robinson in Soho, and BookCourt in Carroll Gardens. I don't know if they're too snobby or what, but their romance selection is PITIFUL. So I end up shopping at B&N even though I'd rather buy independent. (Well, it also helps that there's a B&N on about every second corner. Really, there must be at least 50 of them in the city.)
Oh, and another factoid from the article that I didn't know: 40% of all fiction sold in the US is romance! That's cuz we're crazy rabid readers. No other genre like it. :)