For the most part I agree w W+A, but I did have one "corked" wine that had a plastic cork. Very upsetting.
Otherwise, they avoid TCA better than real cork, but have some problems of their own. First, they aren't as compressible and don't expand as well as cork. So when you put them into the bottle, they never expand to fill the neck. That means they have to be a tighter fit to start. That's OK, but since the machines get jammed sometimes, some producers use a stopper that's just a hair smaller than it should be. Seems to work pretty well, but doesn't really make a perfect seal. Drink those wines fast.
Second problem is that they strip the teflon off your corkscrew if you open a lot of them and if you've got teflon on the corkscrew in the first place.
Third problem is that they don't compost. So if you throw them into your compost pile, and if they're nicely colored - yellow, red, purple, etc., your pile looks more like a garbage dump.
Nonetheless, I NEVER hold it against the winery when I see a plastic stopper. I figure the winery is doing its best to reduce the cost of the wine where taking the cheap route doesn't matter, and also to reduce the chance of contamination. Those are pluses IMO.