The only thing I can add to what Woody said is personal preference.
The combination of the larger wheel and the trail change makes the handling different.
It is subtle in my opinion so it all depends on how in tune you are to your bike I guess. Some may find it uncomfortable others won't notice and others like me could find it better.
For example Woody suggested I get a steering stabilizer and I feel that I don't need one. If I installed one maybe I would like it better, but for now it is just fine the way it is.
I loaned it to a friend a few weeks ago and he didn't care for the street handling, but loved the off road handling.
Unlike what Woody said I find it "faster" rather than slower, but do find the straight line tracking better.
What I notice is a much more "aggressive" turn in (takes more press though so maybe that is what Woody means by "slower") and faster response from the steering on road. I really prefer a "heavy" bar, but I am a big guy so finesse is not exactly a strong point for me.
I relate the front end feel to a mountain bike like response versus a normal old beach cruiser if that makes any sense.
I find it instills much more confidence in the front end. Point it in the direction you want to go and it goes.
Come in a bit fast to a corner and I feel like I have more traction and if it does slip it recovers faster than the wider smaller rim.
Drift the rear end a bit and the front end still steers well.
Front braking has a much better feel to it. I can sneak up on a lock up and use more of my front brake.
The front end just bounces up on things rather than the "banging" like the 19 can.
Again I have no negatives on running the 21"
I am looking forward to when Jimmy Lewis gets done with his write up I am sure it will offer another bunch of good info/opinion maybe even a few more tips for running one.