I've been running in Five Fingers for approximately 6 months now.  I use them for road and some trail.  Apart from the fact that Andy B and my wife absolutely hate the sight of them, it doesn't detract from how good I have found them.  I have changed my running posture so that I run much more fluidly and I haven't had any problems with knees and shins since (which I got from cushioned shoes).  The times just keep on improving.

You have to watch them on stony trails (expect bruised heels occasionally) and if you touch wet grass you'll notice the lack of grip but I absolutely love them.  I have the ones with neoprene uppers which means they do get warm but I don't really mind.  That's far less uncomfortable than how cold your feet get running in snow...

Cost?  Astronomical.  You have to have too much to spend and have a correlating lack of fashion.

Downsides?  The smell (although they go round the washing machine quite easily) and I'm not sure if there is a version to allow me to seriously consider dropping my Innov-8s off road.  The sides of my little toes are also getting a knocking (but no blisters elsewhere ever). And the wife won't be seen dead with me wearing them. 

Advantages?  You can use them for almost anything.  I find it incredibly easy to take running kit wherever I go as they take up virtually no space in your kit.  You can use them anywhere, from sailing, canoeing, open water swimming, low grade climbing...

If you fit the criteria of having money to burn and no style sense, then you have to try them - if only for the novelty/lunacy factor.