I had a Cactus as a courtesy car while the C3 was having a few bits done.
I wasn't too sure about a 3 pot petrol engine after years of driving a turbo diesel (aka Satan's spawn

), but it coped well enough with what we needed which included a fair bit of motorway driving.
Was a bit "thrummy" under load - almost (original air-cooled) Beetle-esque if pushed, but I dare say that you'd soon get used to that and it settled down at normal speed.
The fuel gauge didn't seem overly accurate - the low fuel warning light was on when I picked it up so I put £7 worth in (just over a gallon - I was only having the car for one day) and the gauge then went up to nearly half full and stayed there for the rest of the time I had it (approx. 50 miles). If you believe the trip computer, fuel economy was in the high 40's during my loan which seemed good for a small petrol engine
I really like the external styling of the Cactus and was half thinking about one for our next car, but...
What I really disliked were lack of headroom inside (I'm 5ft 9 tall, so no giant) and it felt quite claustrophobic with the narrow windows and low roof line - now I'm used to driving a C3 with a panoramic windscreen so that won't of helped but the Cactus felt very hemmed in and almost like you were peering out of a letter box when looking out of the windscreen, and I really didn't like the "tablet" screen in the centre of the dash that runs things like the infotainment system, heater / AC, phone etc - it took your eyes off the road for too long to change anything and unlike dials / switches it demanded too much attention when I felt you should be concentrating on driving. That said, it seems like these screens are 'the future' and we're going to have to get used to them as they're appearing in more and more cars now.