I would give KitKat 4.4 a 9 out of 10. For KitKat 4.4.2, I have to give it a 5 out of 10.
Before KitKat, I only use custom roms because they were faster. I started using stock Android full time when KitKat was leaked out for the Nexus 4 during mid-October. With the improvement in performance and ART it really transformed Android for me, Android felt exceptionally smooth for me now and I say this again, but iOS is no longer 'smoother' than Android. I like the new design and simplification of the interface as well (though there are still improvements needed). The difference comparing to custom roms is that it's fast now, but with stability and less bugs and better battery life. It just allows me to appreciate stock Android a lot more and felt less of a need for those extra custom roms features.
Minus one point for 4.4 because I heard lots of people having issues with the OTA update, Google have done a great job at gettings the updates out fast I think, but it doesn't go smoothly. I don't own any Apple products, but I usually run the Apple OTA update on my family member's Apple devices since they never bother to do it,. Unlike me who have more than one phone, it's bad if their only work phone stopped working. Yet I never had a worry that Apple update wouldn't work without a hitch (going from iOS6 to iOS7 even). I don't have issues with Android OTA myself since I know how to flash firmwares, however I heard too many bad stories from people who never root nor modify their phones, yet ran into problems, and countless of post-update issues.
I also didn't like Google decison (or for whatever reasons) that made the tethering stopped working for T-Mobile starting with KitKat.
I'm not happy at all with 4.4.2 updates. Like many others I don't like the idea of Apps Op being removed, I think it's a huge loss in term of control. Also there are lots of incompatibility issues with 4.4.1 and 4.4.2 releases, some apps I personally use had issues, and I'm sure others can name a few. I can't fully blame Google on this since I'm not a programmer to understand how the code changes in KitKat affect things and maybe it's for the better in the long run.
Things Google still have not fix for the Nexus 4 as of 4.4.2:
-stock camera app still terrible, I actually get faster shutter speed and better image sharpness with alternative camera apps
-the issue with wired headset having extremely low mic's gain, people have been complaining about it for a year now, Google just finally is acknowledging it
-I can't be totally happy until Google does something to vastly improve the wakelock issue with Android that many users sufffer from, though it has been getting better