There's plenty of people who have tried them but I don't recall any major success stories. Search the site for lots and lots of previous discussions. There's a number of threads with homemade vortex generators...that's what I'd try, if anything. I like this one: http://www.gassavers.org/showthread.php?t=6178
I was thinking of building some out of frozen water to see if they would have any effect. Just get out on a cold winter day and drip water on the roof to make them. Easy to remove afterwards . . . . they melt.
I haven't seen a difference with VGs. Wasn't any real strict testing, and any noticed changes in economy may have simply been to other factors. Comments I've seen on the airtabs are mostly positive for those with boxy trailers or van bodies. Most feel they improve the vehicle's stability which might make the airtabs worth it regardless of fuel economy effects.
I'm planning on doing string testing and installing VGs on the HHR's hood. I'm mainly hoping to reduce rain, snow, and bug splatter.