The ability to carry a gazillion books without them weighing a gazillion pounds!

Money -- ebooks are free, paper books cost money :D

Library -- ebooks can be borrowed at home, paper books require a trip to the library
.

Space, money, availability and ubiquity.

Space: Obvious. Ebooks don't take up space in the house or in suitcases
Money: Ebooks were and to some extent still are much cheaper than paper books
Availability: I could get the latest Baen SF/Fantasy books as soon as they were published, rather than hoping to find them in a local book store.
Ubiquity: With a PDA, I always had a book or ten with me.

You try to put a copy of the Count of Monte Cristo in your coat pocket... I could do that with a PDA.

Also, reading in bed became a lot less painful as I can easily switch side, without having to hold a thick heavy book on it's side...

Space constraints..I seriously downsized my living space. Plus the nearest books store is approx. 75 miles round trip.

To start with it was the lure of free books (baen CD's, free library, the Gutenberg sites).

Matters of storage space, durability(many of my pbooks are falling apart due to excessive reading while my deDRMed ebooks are still readable), a good method of organization, and ease of access (portable readers with lots of storage and adjustable fonts) are why I grew to prefer ebooks.

Most of you are missing one very important point. It saves trees.
With me knowing the pollution that making any electronics causes I will not say that they are less pollution but at least we save a few trees. I also like caring around dozens and sometime hundreds of books on a device that weighs only a few oz.

Not particularly. Trees are grown to make paper. If fewer books are printed, fewer trees will be grown. "Wild trees" aren't used to make paper (not in western nations, at least).

I tried my first e-book on a dare. It was good so I tried another, and another. I said to myself, "I can stop any time I want to!" But it's years later and I'm still reading them.

(Do you have any good resources for those trying to quit? ;))

Actually cvkemp, not reading paper books does not save trees.
Most paper is made from 5 year old trees. The paper mills replant a tree for every tree they cut down and have since at least 1984.
So they no longer clear cut forests for paper.
Yes, I have personally seen the trees for paper. Also saw the entire paper mill.
Now all these new houses are another story, they do still clear cut for lumber.

Now back to the topic,
I read ebooks and paper books.
Space and money are the reasons. Also an ereader fits in a camper better than a bunch of books.

Actually, eBooks do take physical storage space. A flash drive, a hard drive, a CD, a DVD, blu-ray, Reader, phone, tablet, etc are all physical. You have to have someplace to store your eBooks. And that place is physical.

The main reason I started was because I wanted something easier to read while standing on the train to/from work. 1000 page novels are much harder to carry and read one handed.

The other reasons listed above (space, cost, multiple books in one device, etc.) all played a factor too but they weren't why I changed.

But the amount of physical space does not increase with the number of books. I can store 1 book or 1 million books on my 1TB hard disk, and it still remains the same size.

... a fact which you are perfectly well aware of.

And all those things together take up the space of less than 20 books.

Storage space. My home was one big bookshelf. Now it's only half bookshelves. :D