They're pretty good for their portability, and you can even get "Free" ones, if you wish to find one on the internet.
The store itself offers a lot of books, and supports PDF's, E-Books you've already bought online, and of course ones you would get from iBooks itself.
For example, if you would buy an E-Book software download on "Programming", then it could be easily imported into the iBooks application itself.
Or the iBooks store has books on Software Programming, or basic Programming Languages if you require those.
Or in your case "Image" editing, they have those too, and thousands of other books.
The displaying of the book is pretty well done on an iPad, and with the iPhone or iPod it's a good electronic book replacement for those who don't wish to have a laptop or anything in that caliber on them all of the time.
The app itself neatly organizes all of your books, and with the power if iCloud all of your books can be viewed on any other device that you would own with your Apple ID.
So, is it worth it?
For what it offers, yes.
If you prefer the touch of paper in your hand, and the ability to do more with a "Paper" book; Then the app and buying of books isn't worth it.