Went fine on my iPhone 5 w/ 16 GB.
No problems. Smooth update from 10.2 to 10.3. I skipped 10.2.1.
Only thing I notice is my total capacity went from being reported as 12.24 GB with iOS 10.2 to being reported as 12.22 GB after updating to iOS 10.3. Minor decrease. Probably just the way the new file system reports it.
My available space went from 5.66 to 6.60 GB. But I also had the 10.2.1 update that I kept ignoring. Would be nice if updates you didn't want wouldn't waste space.
So, most of my space increase was probably just deleting the 10.2.1 update package.
Otherwise, all my data is there, nothing lost. Up and running.
No faster, no slower. Just the same as it was.