Combination. I'm a collector, so I want physical copies of the books I want. But I do almost all of my actual reading now on my phone, and to me, it's worth the $8 for the ease of holding my phone vs. holding a book open and the convenience of having access to the book on my phone wherever I go, since the phone goes everywhere I go.
I will say that this was not my position a year or so ago. The convenience factor finally won me over to ebooks. Oh, and as Stoek
mentioned, one of the best features is the dictionary/Internet search feature. I love being able to click on, say, a species name, click on the Google option, and go right to Memory Alpha/Beta and see a screen cap or other picture to be able to visualize an alien character. SO cool!