A very rough guide can be taken from the the percentage of good/bad reviews on Amazon.com, can't it? And from what I see, they don't really suggest that HarryT is that wrong to suggest most people are happy (how many of them are happy with less than perfect screens is far from certain though!):
Kindle 3: 8% - 1&2 stars, 87% 4&5 star.
Kindle 4: 10% - 1&2 stars, 83% 4&5 star.
Touch: 17% - 1&2 stars, 72% 4&5 star.
Kindle PW: 12% - 1&2 stars, 77% 4&5 star.
The PW is clearly not as good as Amazon made it out to be, and there is definite room for improvement, but sometimes you can focus too much on the people complaining and forget that so many people aren't complaining.
I think the wide variation in the screens, as show in the photos's deslock posted
here, are definitely a concern though.