There's a FOTW article some time back by Kelly Glissmeyer on parasols. He's a really good fly tier, and a really good fisherman, and he really likes them.
Personally, I haven't tied or tried them. I prefer to just trail the unweighted nymph off a dry fly of the same "hatch." That gives the fishy two options for dining out.
John
P.S. Here's a link to Kelly's approach.
http://www.flyanglersonline.com/fotw...tw20101108.php