Nice site, although it does have issues with the analysis as outlined by others here.

One suggestion I would have is some sort of filtering based on the content of the tweets. This tweet returned "feeling good":

"New post featuring: @NewLookPRTeam @nextofficial @Matalan @hmunitedkingdom @uoeurope @Accessorize @ASOS http://t.co/nGsFj306xT #fbloggers"

This one returned "feeling average":

"@dannykobe17 @DanielRacheter @Khuds_ @shangambling @Umar_Wilshere19 @_mikenewell_ @BlueKay10 is that ollie?"

Whereas there's absolutely no real sentiment to derive from this sort of thing.