Testing convertibles always causes a problem. As a rule, cars lose structural rigidity when you cut the roof off, and therefore drive less effectively.
Simple - but the sort of person who wants a drop top is seldom the sort of person who gives a toss about handling. Or ride quality. Or annoying rattles. Or looking like a bit of a twat. (The views of this road tester do not necessarily reflect those of the BBC etc.).
The latest 3-Series Convertible is one of the best of them. The new folding hard top means refinement is better when it's up, and it's not a scuttle-shaking fiasco when it's down.
But, and here's the rub, it's still nowhere near as good as the coupe. Whatever engine, whatever wheel size, tyre choice or CD selection, the fundamental excellence of the 3-Series is unavoidably diluted.
Following the US launch (flat, straight roads), we drove the 3.0-litre diesel in the UK (bumpy, windy ones). It's still fine, good even, but the ride isn't great, there are squeaks and shakes around the cabin, and the handling is less precise than in the coupe.
So it's a slight on BMW's latest 3.0 diesel powertrain to put it somewhere where it can't shine.
If you like BMWs for what they do best, don't buy a convertible. But if you're set on buying one anyway, then I doubt you got past the first paragraph.