For me it's a matter of space. I spend about 10 months a year on ships and I have to take everything with me that I'll need for 5 months at a time in 1 bag. I have my big laptop for gaming and pr0n and the like, but it's a lousy e-book and doesn't get online. I have an e-book reader that is slow, b&w only, has issues with different book formats, and has a small screen. When you read 100+ pages an hour, paperbacks are just not feasable. I also have a phone that I can check my e-mail on when I get into the US ports, as long as I don't want to be able to reply.
For me, the tablet is 1 piece of tech that I can use for reading multiple formats of books that's easy on the eyes, a web browser that's big enough to view the whole page, an e-mail terminal that's comfortable enough to type on, yet something that's light enough and small enough to use in bed and doesn't take up valuable sock and jeans space in my sea bag.
My biggest limitations with the iPad are the lack of expandability in the memory (which cuts down the number of books I can load), the lack of a PC interface (You can only transfer approved file types via iTunes), and the market (I like to have options that the Apple store just doesn't have).
So, with all that in mind, I've been anxiously awaiting a good, 3g, 10" android pad. Unfortunately so many of the potential winners either don't have all the features I need (3g, Android market, capacitive touch screen, etc.) and the Samsung tab is becoming a bigger and bigger pile of lose as we get more info on it.
Slate PC's are just to big, full-blown tablets are too thick, hot, heavy, and power hungry for my needs.
But that's just me.