Wikipedia is surprisingly informative:
http://en.wikipedia.org/wiki/Tentacle_erotica
There is definitely more touchy-feely type tentacle porn (pun ENTIRELY intended) but most of it is non-consensual and has an aspect of the horrific or terror about it. Think "Legend of the Overfiend" (my first exposure and a great movie, though it hasn't aged that well.) I think the attraction is, if one man with one cock can please a man, what can one creature with 1000 cocks do?
Plenty of crossover with other porn types (bukkake, forced, groping, etc)
There are live-action versions, as well as a subfetish (or related fetish, your choice) of putting squids and snakes in ladies bits. It comes from the Japanese, so you know it is deliciously perverted.
I can appreciate it on an intellectual level, and find it quite titillating, however it's not something I would seek out as it's just not THAT big of a deal for me. Still, some of it is quite erotic. Fun stuff!