Loads of fun, you should not hesitate to buy one (Halfbike 3)!
At first, balance is not too difficult, turning is okay if slow and wide, and most time is spent in 1st and second gears, cruising slowly. Immediately I wished I had ordered the lower gearing, because I could not fathom how I would ever possibly ride stably in high (4th) gear. Even in 3rd gear, it seemed incredibly unstable and wobbly.
A few weeks later, and way more experienced, I now have excellent balance and control, sharp turns are a breeze, and I barely ever use 1st gear except for climbing hills. Fast cruising in 4th gear is now smooth and stable and fast up to 25mph (40km/h)!
Why is it so much fun? For me, riding the Halfbike feels just like my first bmx bike as a kid, where most rides were spent standing out of the seat while pedaling furiously to go faster. Feels like being a kid again!