I saw ~6% jump up in my mileage when I put them on -- they definitely helped. I am now going to sell my alloy wheels...
6%...whoa!
Let me try some math...
At 60 mpg that would give me 3.6 extra mpg. On a 10 gallon fill that would be an extra 36 miles per tank or roughly $1.25 saved each fill up (at current gas prices). To break even on $70 discs I would need to fill up 55 times (that's about 2.5 years of fillups for me to break even...probably 3 years knowing that 60 mpg is not realistic in the winter...hmmmm). They look cool...but 3 years is a long time to break even...no?
I wanted to pour my first couple months savings on gas (switching from 4runner to Prius) on a mod of equal value...tempting.
I have the Moon-brand discs, and a bead of caulk around the inside where the fasteners meet the disc quieted them right down. Prior to that, it sounded like I was dragging around typewriters.
Moon discs are a bit pricey (I paid $130 or so shipped), but they will not fall off, ever. They fit so tight, actually, that I need a clawhammer or crowbar to remove them. Plus, they're aluminum, so they're fairly light. They scratch 'n' dent easily, but do well with a coat of wax every so often.
Interestingly, I originally bought them because they look awesome, but the tiny aero benefit is a plus.
I forgot that you mentioned that before. I don't agree with the fall-off part, but I wonder if the caulk made 'em stick better.
I bought mine (16" stainless) ... lost one on the way back from a trip to Pennsylvania. Now I don't know whether to buy a replacement or give up on them and try a different hubcap design.
I can't say that I've noticed a gain in MPG ... but then again, I haven't really run enough in the warm weather so I can't make a direct comparison with my hypermiling last summer.
I actually found on of these by the side of the raod ... but it was a 15"
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Now I don't know whether to buy a replacement or give up on them and try a different hubcap design. 
