Back when we got ours, in 2007, the short list of cars we considered was the A4 Cabriolet, and the VW Eos. The BMW 3-series ret. HT wasn't quite out yet, but I didn't think the extra money it was going to cost was worth it. The Eos was reasonably peppy and drove nicely, and cheaper by a fair amount, but it just didn't have that much appeal - and in retrospect there have been problems with them, including roof leaks (at least in the early years). The A4 Cab was quite nice - we were looking at the 2.0T version, which was similarly-priced. I kind of liked the idea of the retractable HT, though, so the cloth top of th A4 was a slight negative. I am guessing that with the A5 Cab, prices have escalated a fair amount, making it non-trivially more money than a C70.
My list today would add the Infiniti (what is it, the G37 convertible?), and perhaps the Lexus IS convertible. Both of these are also a bit more money, I think, but seem pretty nice on paper.
The interior of the C70 is a bit plain, but I like the somewhat spartan Scandinavian look.... I think it is still true that the C70 has the most usable back seat of all of the above cars - still tight by sedan standards, but more workable than the others. Even if the others are quicker, I view the C70's power as adequate (even with the Geartronic), the handling quite good (within the confines of a FWD set-up), the exterior design is very graceful, classy, and sleek, and the interior design as reasonably nice in a low-key way. I kind of like the vibe of the car, as a somewhat "sensible" convertible. Our C70 is my wife's primary car, and there may be a slight amount of "chick car" aura to it (not as much as with an Eos, though...) But I am very happy when I am driving it (perhaps helped out by the fact that ours is black....)