It depends on the book, and depends on what you need to do.
I find eBooks a proper pain if you need to go back and fourth between a select set of pages. Theres no convenient or easy way to 'glance' on one page and then quickly return. In fact, you normally can't return at all. You can setup bookmarks, but the process is much slower and clumsier than done with a traditional book. You also cannot scan pages anywhere near as quickly when using an eBook versus a traditional book - for when you need to find a section of text (or a table) of which you are not certain its exact name or placement in the book in question.
eBooks due to their portability do work well though if you mostly need access to a single or specific section(s), where jumps are small or non-existant, or for sequential reading.
For fictional literature, eBooks are convenient. For learning materials, they're often poor.