The engine does have its upsides for driving instruction (47mpg) but seeing as BMW recently uprated it (now 88BHP), even they must have thought it the weakest link in the cars armour.
You'd understand if you tried to do what I did about a week ago...
...returning home from Derby, I joined the A38 from a large roundabout. There was a slow moving crane (about 20 mph) in the left hand lane so I had to use the right (not the best idea normally because the road takes a sharp turn and if you'd driven on it you'd know it can be dangerous with people coming off the roundabout in the fast lane, realising they haven't got the acceleration they need and swerving back to the left), I was still in fourth (6 speed gearbox) with a transit van (typical "white van man" driver) trying to park in the boot of my dad's car. I gave it the boot and held onto the gear before selecting fifth.
Now, with a modern diesel (turbo or commonrail), you'd expect that if you kept the revs high enough (i.e. holding onto the gear before changing) then you'd be able to ensure that it stays in its powerband, allowing better acceleration in the next gear. It didn't happen and even though I held onto fourth, when I changed into fifth the change in pace was so dramatic that I think the transit driver needed a change of underpants. The car struggled away trying to accelerate with a queue forming up behind us. A discussion later on with my dad revealed that I'd made no mistake, he said that the car was normally "like that".
As well as this, on a cold morning especially, if you don't give then engine between 2,000 and 2,500 revs (remember it's a diesel so that's quite high) when pulling away from some junctions, it WILL leave you stuck in the middle of the road when the next car appears around the corner.
The engine's also overly loud and compared to the petrol, very unrefined. I've never heard this on another MINI but from the outside of the car our air conditioning fan sounds like a Harrier AV-8 performing VTOL when the car's in a car park or stationary at lights.
We did test drive the petrol (and at 90 BHP it's not even as powerful as the AA's Ford Focus fleet) and it's much, much better. The diesel's only saving grace is it's economy, there's NO WAY we'd have bought it if it didn't return 47mpg (official figures are 51.5 combined) for us.
When you come to learn to drive you'll realise that what a learner needs isn't an underpowered car, it's an easy-to-drive car. There is a vast difference. But then again I wouldn't expect you to understand what that means at 15...
The MINI is my dad's only car (my mum drives a Corsa 1.4 SXi) and so yes, I will be in it quite a lot. Don't get me wrong, the engine isn't "bad", it's just nowhere near as good as the petrol.