I'm coming from a SG3, so this is a pretty big upgrade for me.
PROS + Great screen. Reading websites definitely better compared to my SG3. + Love the integration between the new calendar, maps, contacts and google maps. + Lock screen notifications appear in lock screen when I lift the phone. + Still playing with the priority alarms, but I love that I can silence certain apps/games (Clash Of Clans I'm looking at you) at night. + Switching between apps/websites is SO much easier. + Audio sound quality (music, videos, etc) is great. Love the speaker placement on the front. + Love that the phone notifies me how long it'll take to get 100% charge. + The animations/color schemes on simple things is a nice touch. When you select a contact card and scroll up, their name flows into the header. And that card contains a lot of useful information like most recent chat messages and calls. I dig the placement of that type of information. + Back to the calendar, I like the automated thumbnails that are specific to the event. Or when needed, the map of your event destination.
CONS - I was using the Nova Launcher on my SG3 and I loved inserting my own icons. I'm trying to stay with the default google now launcher but...I HATE the default icons. - Dislike that the phone itself is not flat. Sitting on a table the bulge on the back makes it move and that's annoying. - Size becomes apparent when talking for long periods of time. - Talk volume, even at the highest setting, is still low. - Kinda upset Swiftkey (3rd party keyboard) doesn't have similar haptic feedback like default google keyboard.