An arsonist described the sensation of watching a fire he'd started on the Jeffrey Donovan production "Touching Evil" as a million ladybugs in cashmere high heels crawling all over underneath his skin. That's a fair beginning of a description of tickling. I think FFT32 and Raystanz333 are also heading in the right direction. Not for everyone, but for me and probably for most fetishists there's a pronounced sexual component to tickling.
The experience of your very body betraying you, of needing to climb out of your skin, to get away from this maddening pleasure, energy released by a thousand nerve endings pouring unprocessable electrical information to your brain. Again, that's merely a description (limited here by the word "unprocessable.")
Essentially, your brain is dealing with a confusing storm of information regarding body position, interpreting touch, and a maddening need to anticipate something it cannot anticipate. At the same time, neurochemicals related to euphoria are being released and generally you come out the other end of the experience feeling fantastic, although the "during" part will probably feel something like an uncontrollable roller coaster ride.
