There are definitely things to like in Lightning, such as a more coherent listing of objects in the Setup (eliminating the split between Customize/Hunt in non-alphabetical listing, avoiding Contract when you mean Contact), listing of internal field names for built-in objects, the created by/last modified by information staying visible while editing records, etc.
The biggest problem I find when using the Lightning UI, and the reason I switch out of it for any serious word I do, is the amount of time for it to load combined with if re-loading for a new tab. I regularly open several tabs both during data verification as well as during admin tasks. It's so painful to do so with the Lightning UI that I usually switch back to Classic quickly. Having done that, I don't often go back to Lightning since I don't really want to sit there while an animation is playing during the load.
So in some sense, a comparison to Windows 8 seems apt. Windows 10 kept the "Apps" of Windows 8, but it also introduced the idea that they could be displayed in windows (!), and other developers developed the possibility to bring back the regular Start Menu, and I'm pretty with it (setting aside the privacy concerns for this discussion). I hope that Salesforce can get the new UI finished, more consistent, and find a solution for the multiple tabs problem.