I have a recently converted, 16 year old 3 speed Brompton, its fitted with a Swytch kit.
It works and rides very well, I keep it folded up by the front door, so its easy to get out and go round the shops etc. For visting places its advantage is that you dont need to leave it outside somewhere and get it nicked, you can fold it up and push it around like a shopping trolley. Replacing the standard small wheels rolling with larger skate type wheels helps a lot here. And of course its the type of bike you can easily take on a bus or train or chuck in a car boot.
One sort of issue with the Swytch kit is that the controller\battery is on the handlebars, so you have to remove the battery pack, which is easy though, to fold the bike. I am going to try mounting the controller\battery on the front carrier block so there is no need to remove it when folding, but its likley the assist buttons will then be out of reach when riding, not sure if thats going to be an issue.
I did consider buying a new electric Brompton, but at circa £2750, they aint cheap. Sure you would get the buzz out of riding something so flashy around, but my old Bromton came up quite well after a complete strip down and clean. I did need to replace the cranks since the chain rings were worn and there were no replacements. My converted Brompton does not look so flashy as a new electric one, but I have the same thing basically and saved circa £1800.
If something went wrong with the Swytch setup, I would consider the Whoosh kit, but I would probably want to see one folded up first, not sure where the controller thats in a bag of some sort (that you need to supply) attached to the handlebars ends up.