Hi Jeff (or is it Cool?),
I came from an HTC M7 to M8 and then to the G4. I've had iPhones and the Samsung S2, S3, and S4 in the past. The reason I went with HTC in the last two years is because their UI had the least lag of Android phones with the iPhone having the least lag and smoothest UI (but this is not surprising as they only have one hardware system to deal with an an ultra optimized OS).
The big sacrifice I made when using the HTC is the camera. My needs changed and I needed a good camera so I decided to try the G4. In my experience, the biggest disadvantage of LG is the software. I could be wrong but LG actually stands for L(a)G

So, in the end, I traded the smooth UI and bad camera of the M7 and M8 for the great camera and questionable UI of the G4. That said, it is something I can live with for now.
Battery life doesn't seem to be a problem (especially after 4 to 5 full cycles in the first week... I've noticed this helps a lot). I actually purchased one G4 a couple of months ago and returned it because I wasn't sure if I wanted it. The first one I purchased had some heating issues (but like you said, it was only the top of the phone). It wasn't a problem because it never shut down or anything.
Surprisingly the unit I have now has no heating issues at all which is weird.
But, as I'm sure you know, the biggest downside to the G4 is the touchscreen responsiveness. It misses a letter every now and then and interprets a swipe as a tap and ends up opening up links or apps when you mean to swipe. Basically, you have to me more deliberate with your taps and swipes.
I've noticed that people's perceptions of lag on a phone vary considerably so this is something that you just have to experience for yourself. Another thing I've noticed about lag and UI smoothness is that you can't get a good picture of it from watching professional phone reviews (besides AC reviews of course

. The professional reviews are always after a week or so of use and not after a couple of months of use with 100s of apps and a reasonable filled cache. My experience is that phones always slow down some after the first month of use. Right now, I do a hard reset on my phones every 3 to 4 months or so. It sounds like a lot of work but I actually enjoy it and usually install a different launcher so it feels like I get a new phone every so often.
Let me know if you have any more questions. The camera btw is awesome (video and still). The screen for me is the perfect size. The leather feels great in my hand and the free battery and charger help me because I travel a lot and routinely spend 16 hours flying and travelling around. Oh, and the buttons on the back of the phone are so awesome. I don't think I can ever go back to a phone with buttons on the side or in front... that's so last year
