A lot of the shortcomings listed in the previous posts are more or less accurate. GM cars had plastic GM interiors, the car feels smaller than it actually is inside (on any car you can't get a realistic assessment of interior space until you loosen up and slump into the seat plus adjust it to give you the most comfort.
And like any car, they were designed to hit a medium as far as suspension stiffness and ride comfort go, so potential buyers wouldn't get turned off and leave without buying.
But underneath all that, the cars are an excellent platform and the turbo LNF engines have enormous potential. If anyone is that interested, I documented the improvement path in detail on my car at http://www.solsticeforum.com/forum/f43/coupe-378-mod-thread-102178/
Briefly, I own one of the Solstice GXP coupes, of which 781 were built, only 281 of which had manual transmissions like mine. They were 260 bhp/260 Tq stock, 290 bhp/340 Tq with the GM approved available tune, and weigh right round 3000 lbs. I did the handling fairly comprehensively with new shocks, springs, alignment and sway bars, all of which transforms handling but would have made them unsalable to most people. I also bumped my engine output up to 375 bhp/375 Tq, which makes for a comfortable touring sports car that gets 30 mpg when you keep your foot out of it, but will break into the 12 sec. range for the 1/4 mile when you want to.
The reason more aren't competing is that rules aside, many owners were not young kids, they were older folks wanting to get into some sporty summer driving but for whom the Miata didn't quite make it.
I enjoy mine a lot (right up there with my other modern sports car, a BMW Z4MC).
With my previous modern sports car, an 88 Fiero GT with a 300 bhp turbo engine:
Shown in good company:
Engine bay:
There is something to be said for an engine that can be easily made to put out 200 bhp/liter without going inside it. To put that in perspective, the C7 Corvette with supercharged 755 bhp LT5 engine only had 120 bhp/liter. I anticipate keeping my car for a long term ownership.