I love FON - in theory. I nearly signed up, but then realised I would be one of two users on the Isle of Wight, the other user being in a housing estate in Shanklin; not desperately where I'd want to hang out with my iPhone.
If FON had hotspots along beaches, in parks, in business estates, city centres, train stations etc., I'd jump at the chance. But the intrinsic flaw in it's business plan is that these are exactly the places that it doesn't have hotspots - as they're not residential.
I suppose BT Openzone is a start - but I'm not entirely convinced, as I'm meant to have free access to them as part of my O2 contract, but have yet to figure out how to achieve this - every time I've tried, it's insisted I log in, or sign up and pay. Hmm.