Here's my two cents as a newbie to android with the Nexus 5...
I switched from an iphone 5 to a Nexus 5 and couldn't be happier. I kept my i5 around as a safety net in case I didn't like the N5 but the i5 is now for sale since I'm loving the Nexus. I've owned an iphone, iphone 3G, iphone 4 and iphone 5 and never once has any android lured me over to the dark side since becoming part of the isheep flock all those years ago. The Moto X almost got me but it took the 5" screen, crazy off contract price and other specs of the N5 along with the fact that it runs pure android and the latest revision of it to get me to actually buy one. If apple puts a 5" screen on the i6 I may go back but if google steps up on the N6 (or whatever it will be called) and keeps evolving (which odds are they will of course) I'll stick with Nexus phones. The odds of me owning any android that isn't running pure android or close to it like the moto x however...zero. So it's either pure android or ios for me, none of the skinned, bloated crap.
It was an easy transition for me since I use gmail, google calendar, my contacts are in google and I also use google drive so the only trick was my music in itunes but that was easy since all my music is now DRM free thanks to itunes match so I just dragged and dropped the folders onto the N5 and was good to go. The rest got uploaded to googles music cloud (since it's free for 20K songs) and it was a painless transition. I only lost one app I used often but I still have it on my ipad air so it's all good.
The only slight negative for me is the 32gb max storage, I came from a 64gb iphone so I had to trim my music quite a bit but that was no big deal really.
As for the transition to a new OS from years of iOS, that was no problem since it's pure android and not a convoluted skinned mess like other android phones out there that you couldn't pay me to use. Hardware wise I still reach for the power at the top and a home button on the bottom center to wake it up now and again haha, but as the days go on that gets less and less and I'm 95% used to it now and loving the big screen.