I have grown up the baby of my family, trust me when I say I have been on the receiving end of many a tickling. All through high school and college I have racked up hundreds of tickling experiences, and I am hard pressed to find one that wasn't fun.
In most instances it is one of the primary forms of communication between parent and baby, kids find it a delightful way to interact, and it can be one of the most effective forms of foreplay for adult couples.
As far as fetishes go, this is one of the least threatening, and most pleasurable. Now don't get me wrong...it can be torture as many hardcore pain fetishists will attest. But ask my wife what is THE main way she shows me she loves me? She tickle tortures me every day. It is definitely fun for her, and I have to say (however reluctantly) it is also fun for me.