RE:what fly line for flats fishing?
I own a lot of lines but haven't used anything other than Airflow Intermediates on Monomoy for several seasons. I'm more infatuated with the fact that they are clear than the fact that they sink. As Aaron has witnessed first hand, the slow sink hasn't been much of a benefit this season as it seems that every shot on the flats lately is a last-minute chuck-n-duck. I do think that when you get a good shot and are able to lead a cruising fish by a reasonable distance, then having a clear line below hanging below the surface helps -- it casts no shadow on the bottom. I also think that it's very beneficial when throwing flies into pods of milling fish -- the fish seem to cruise over the Airflow as opposed to around other lines. I also feel like the intermediate lines create a better/more direct connection with a sinking fly -- there is no "hinge-effect" as with floaters.
I've been using long leaders lately (greater than 10'), which is a real headache when throwing heavy flies, so this eliminates much of the effect that the line has on the fly.
I definitely prefer intermediates or sinkers off of the flats and for tunoids.