The Miata is running great.
The original soft top needs to be replaced due to normal wear and tear (canvas separated from support/roof bar, which makes a flapping noise, and the rear window is hazy/scratched up with swirls).
Mechanically, no problems to report, but this was a low mileage buy.
This is the first Miata that I have owned. The transmission is smooth with no hiccups so far. Clutch engagement point is very predictable, and getting into and out of gears has a very 'mechanical' feel - a solid click, and you know you're in gear.
I have the soft top option, and at highway speeds, I noticed flapping noises initially, but over time I got accustomed to them, and now have tuned them out. - Update - After applying weatherproof 3M adhesive to the top (remember, using less is more) the top is secured to the frame top. The rear plastic window becomes brittle in cold conditions (~40 deg F) and will crack easily if one does not undo the zipper prior to opening the top.
Can't comment on the stock audio system since mine is aftermarket (who needs a radio when you have the glorious inline-four revving to 6k rpm?).
The previous owner removed the power steering so handling is very 'tight'. At any given moment, I can feel the camber of the road - every nook and cranny - but it also becomes a herculean task to park, since there is no power assist and I normally park at slow speed.
Haven't had a chance to take this to an autocross/track day, but believe you me, I will have come summer.