I've been curious about this for a long time. I especially hate reading a book with a cliffhanger, and will usually avoid them. If the author warns you there is a cliffhanger then I might give it a try, depends on the author. Too many times though I can only go by what's on the cover. I have to admit that when the cover says book 1 that I expect some kind of ending, even if there are future books to follow. However, when the cover says part 1 I expect a cliffhanger.
I read a novella the other night and the damn thing just ended. No warning or anything. Book 1 was on the cover of this 27 page story. So I did a little investigation to discover that there were 4 other books to this story, all with around 30 pages each. Now, the author was only asking 0.99 for each story, but I can't help feel it's a crafty way to get the reader to pay more.
Personally I would rather the author wait, write the whole story, and then ask one price for it. But my question is, what do you consider the difference is between book 1 and part 1?