I'm really sorry to hear that you're so dissatisfied. Despite the yo-yo, which really bugs me at times, I still love my mini - it's easily the most enjoyable drivers car I've ever had (and between my wife and me, we've had some good ones (Golf GTi Ml 1 and 2, MX-5 1.8 Mk1 and 2, 205GTI and 106GTI).
If it's any help, try my tips for minimising yo-yo:
1. If you haven't done so, get your dealer to update your ECU software - later levels (CD38 onwards) were better and have cured it almost completely for some people.
2. Drive as smoothly as you can - be especially careful to match engine revs accurately as you let the clutch in when changing gear and try to move the accelerator pedal smoothly rather than in jerks - particulaly when switching between acceleration and deceleration.
3. Always use super unleaded petrol - Shell Optimax if possible (it's a higher octane rating than other super unleaded petrol). I definitely feel the car runs better with it and although I haven't checked this, others have claimed that fuel consumption improves enough to offset the higher price.
Have fun - if you can!
Alan