About the lack of trunk space...it's problematic if you expect your two-seat convertible to be practical. But a LOT of people who buy two-seat convertibles use them for sunday drives and the like, and have a sedan/SUV/whatever for grocery runs. It's not a deal-breaker issue for that sort of driver. And let's face it...even the most practical rear-wheel-drive two-seat convertible is pretty darned impractical compared to just about any other type of car.
While the Miata is more refined due to its long history, a lot of people (including me) find it very unattractive to look at. And frankly, looks matter a lot in this genre of car. Also, I've read one or two shootouts that say that the Miata is more agile, but the Solstice/Sky feels more planted and stable. There's advantages to each type of handling, and to a certain degree it comes down to personal preference. I've met people who have test-driven the Miata and didn't like it because it felt too insubstantial. The Miata isn't for everyone.
The Miata might always be better on the track...but how many roadster buyers take their cars to the track? Five percent or less, I'd hazard to guess. I suspect closer to one percent.
While I acknowledge that the Miata has several serious objective advantages over the Sky, the Sky just stirs me deep inside in a way that the Miata just doesn't. I know I'm not the only one who feels that way about the Sky. It's subjective attractions will keep it selling well for years to come, despite certain criticisms from track-day racers (and armchair-wannabe-track-day-racers).
Oh, one other thing that the Kappa twins can offer that Mazda doesn't:
a 400hp LS2 V8 shoe-horned under the hood for an additional US$20,000. Crazy, yes, but man would that be cool
