Hello,
my 2 pence: I have bought three A2s during the last three years, two for me and one for my wife (we keep her A2 in our house in Florence) and I only looked at cars with the OSS. Two of them were working well; in fact I bought the 2nd one quite cheap because the seller thought that it was broken and he had been told that it was very expensive to repair, whilst some careful 'enouragement' and lubrication made it work flawlessly. The third OSS was broken for good (as specified by the seller, but this time he was right!) and it was an expensive mess to have it working again, as you can see in my site my mechanics had to dismanlte most of the nterior to fix the broken sliding part.
I must say, however, that even now the first thing I do look for when I'm looking for a possible 4th A2 is the OSS; I wouldn't buy an A2 without a (possibly working) OSS. Here in Italy it's very nice to have it fully open, just pottering around town or driving on the hills around Bologna/ Firenze or at the seaside. It's not a convertible, of course, but I subscribe to the experience that it seems a bit more quiet than the tin-roof A2s I've driven before, rain being effectively silence by the glass panels.
Cheers to you all,
Stefano