I got a G4 to replace my G3 on launch day. The only ones they had in the Verizon store were either the white ceramic back or metallic back. I went with the metallic back. I really wanted the black leather, but I usually use a case anyhow so I figured it didn't matter. Build quality is very similar to the G3, but the chrome band around it looks better then the silver plastic one on the G3. The best improvement in design is the slight curve of the phone. It's more comfortable to hold and talk on in my opinion. I know it's not an all metal unibody like the S6 or iPhone 6, but having a removable better and SD card does limit some design choices. I think LG is smart to go after this market since everyone has left it.
Performance is very good and I don't notice any lag with the stock launcher like I did on the G3. LG has polished up the UI and it looks and works well. The cameras on this phone are excellent! If you like to take selfies you won't believe how good you look in the forward facing camera! Battery life is decent. I'd say it's about the same as the G3. The display is much improved over the G3 and the colors pop out much better.
Last, LG has a great deal until June 30th for getting a free extra battery, SD card and charging stand. If you get a G4 you can fill out a rebate form for these extras.
Overall I'm very satisfied and although this phone will never sell as well as the S6 or iPhone I believe LG was smart offering an alternative that is actually different. There are plenty of Samsung fans that don't care for the sealed S6.
Edit: I wanted to add one thing... If you are a Verizon customer the good news is they did not put the Verizon logo on the front glass like they did on the G3. It makes the front look a lot nicer in my opinion.