Above all, the Brompton is a lot of fun to ride in urban settings. Its powerful enough to breeze up hills with near zero effort, but feels nimble. As with any Brompton, you probably won't win a half-mile sprint on it, but thanks to the pedal assistance, you most certainly will get off to a flying start. The reason it stands above other electric bikes is that Brompton has worked out how to apply power assistance to your pedalling so it feels natural. It also doesn't feel so much like it's trying to fight you once you reach the maximum, 15mph assisted speed.
After you decide which style of e-bike you want, consider the class. In the US, there are three classes defined by the type of assist and how fast the motor will propel you. Most electric bikes sold are class 1 or 3. Class 1 bikes have a motor (max 750w) that assists while you're pedaling, up to 20 mph. Class 3, also known as “speed pedelec” can also have up to a 750w motor, but it can assist you up to 28mph. Both of those are allowed in most states and cities without license. Class 2 have throttles that don't require you to pedal to get a boost. They're allowed on most streets, bike lanes, and paths, but less popular than the other classes and not covered much here (because we still love to pedal and the greater distances pedal assist bikes can cover).