I used to borrow one quite often, and it was great fun! This was an underseat steering one, with an optional fairing. I forget the maker.
Plus side;
Comfort
Speed
Fun
Grin factor
Ability to cover immense distances
Negative side;
Not too hot on really steep hills (No ability to get out of the saddle and honk it up!)
Visibility. It's an issue, and no mistake. Get something up in the air like a flag, or a small light LED somehow and use it at all times.
Storage and portability – They're great when you're riding them, less so when you're off them! Rather cumbersome!
If you've got room for one, and live somewhere with decent, not too steep roads to ride on, they're great fun! I still have a hankering to have one for blasting around Salisbury Plain. Am thinking something with a tough touring type tyre which could take on the fire roads as well….