"Can you do decent speeds? Can you go up a reasonably steep incline without toppling over sideways from lack of speed? "
I find that a combination of light weight and gearing make small wheeled bikes go up hill faster on smooth surfaces. The smaller wheel base, and high center of gravity, on my Birdy and Strida cause them to 'wheely' on very steep hills. Downhill is more of a problem. Going fast downhill on a small wheeled bike can kill you as the bikes are less stable, and the front wheel gets caught in smaller holes. Do not race against larger wheeled bikes downhill. The Birdy front suspension is actually a safety device in my view, not as safe as a larger wheel, but better.
I used my Birdy on many 40+ mi rides and found it comfortable.