I've had two.
First one was a 2004 Cooper S (so we're talking a couple of generations ago now).
I really liked it, very fun to drive. Handled like a skateboard (a bit like the old-style Issigonis Mini). It had a supercharger. The interior looked great when new but did start to wear quickly. Ditto the driver's seat. Not comfortable on long journeys, and noisy and crashy because of the sporty ride. The boot and back seats were tiny. Best thought of as a kind of upright coupe, really. Mine had an awesome Harman Kardon sound system but that was a cost option so they don't all have it.
Second one was a 2014 Paceman. Also Cooper S. A bit of a pointless car really, it's kind of a three-door coupe version of the Countryman. Mini didn't sell very many! It was similarly impractical, but less agile as it was sort-of a crossover. Slightly better ride than the hatch. I thought it would be cooler than it actually was... This one had some issues with losing power and the electrics being a bit dodgy. It got fixed in the end but I'd lost the love by then.
Ownership experience was OK. Mini almost never discount, and they charge a lot for servicing (they're BMWs, basically). The dealers can be a bit Henry Wadsworth (when they're good, they're very very good, but when they're bad, they're horrid).
They weren't the most reliable cars I ever owned but they weren't the worst either. Get one in the right colour and spec and even the older ones still look really good, in my opinion.