I've enjoyed reading this thread, thanks for getting it going.
I'm fortunate to be able to get to Europe a couple times a year for budget travel 2-3 weeks each time and most recently returned earlier this week.
My usual routine is to fly over then use the trains to get around and then fly home from a different airport than I arrived etc. I get to big cities, small ones and anything in between.
I'm definetly a one bag guy, a very light packer and over the years I've used all sorts of different small bag types.
For my October 2015 trip I borrowed a friend's small, new, somewhat low to medium end, hard shell spinner (or whatever they're called) to see how it would work for me. This thing was pretty small to him and he laughed at me and mentioned that I'd need two other bags if I was seriously going to Europe for a few weeks. I welcomed the challenge.
Packing it before I left was very easy and using it throughout the trip whether in the airport or to chase metros, buses and trains was an absolute breeze. Cobblestone streets, sidewalks---it was all remarkably simple and very enjoyable.
As a seasoned traveller I was actually pretty surprised. I half expected to leave wheels and zippers and who knows what else all over Northern Europe during the trip but instead it was one smooth ride after another.
It definitely won me over and I'm now spending my weekend looking online for a decent one for my future European junkets.
As many others have mentioned, this whole debate/dialogue is pretty personal; we each need whatever it is we each swear by. My only advice is to use what works best for you but not to close doors you haven't yet opened either.
So if anyone knows of a decent, smallish, hard shell spinner that isn't very pricey and that'll be primarily used for European trips, I'm all ears.