I've had my Nexus 5 since it's release. It's been amazing. Used it heavily for an entire year as my primary smartphone. At the time Android 4.4.4 was my favorite version. One month after the Nexus 6 was released I took delivery of mine.
Kept the Nexus 5 as a backup since it was too good to sell. Also kept Android updated on it. Upon the release and arrival of my Nexus 6P I decided to keep the Nexus 6 because it too, was excellent and I learned that 6.0" is the perfect display size for the way I use my smartphone.
Currently I've spent some time with my Nexus 5 since upgrading to ver 6 of Android. Other than being a bit small, I'm very pleased to say that battery life is about 10% to 15% better. An amazing experience considering its age.
Now that all three Nexus models are running ver 6, I couldn't be more satisfied. My iPhone 6S Plus isn't bad, but most definitely not as smooth and lag free as its Nexus counterparts.