I test drove one of these from my local Audi main dealer a few months ago.
It was a 2003/03 plate 3.0 V6 Tiptronic with 33,000 miles on the clock.
I was left feeling very underwhlemed - the first thing that struck me was that the interior had no worn well and was looking rather tired. On the move, the cabin creaked quite a lot, not something I've experienced in any Audi before or since, so I reckon its related to the additional bodyflex of the cabrio. The salesmen kept trying to excuse this by pointing out it was a used car, not a new car.
The last A4 I'd driven before that was an older 02 plate 1.8T SE Saloon. It exhibited none of the creaking that the Cabrio did.
Performance was so-so, didnt set the world on fire, infact to be honest, the only thing it really did well was look flash. Handling was stodgy, steering very light. Roof was quite funky.
If you are bored of an Alpina B3, an A4 Cab is absolutely and totally NOT what you need to replace it with. Unless you only buy cars to pose with. The Alpina is a much better drivers tool. A regular 3 Series infact is a much better drivers tool BUT I have not driven a 3 Series cabrio only the Coupe and Saloon.