I bought them out of curiosity more than anything else - and was very thankful I already had ordered some ER-6i's.
The are a great little idea, but just don't work in the way I thought they would. For one, the earphone now has to sit outside of the ear due to the thickness of the EarJam attachment, and because of this it just won't give a true seal with the silicon buds. The fit is just terrible.
Which is a shame as audibly they do exactly what they are supposed to. The bass gets a huge boost, slightly at the cost of treble which gets slightly tinny, but acceptable.
However I do have to totally disagree with rbeau - they are in NO WAY a subsitute for a highend set of earbuds - and even Griffin don't make the mistake of making such a bold boast. If you don't mind the imperfect fit, and the oddity of having your Apple earphones being worn outside of your ear, you'll be alright. I got mine for £0.01 on eBay (obviously plus p&p took it up to around £5) and if you can get them for this much or cheaper they're worth a quick look.