I have a couple fo serious gripes about windlight, however I'll start with the good.
AWESOME. I've always wanted this in SL, and since the firstlook windlight wiewer was released in june, I've anticipated its comeback. Awesome job. great reorganization of the graphics customizations. Awesome world options. One not mentioned change is the fact that inventory landmarks, when double clicked, now cause you to teleport rather than open the landmark. I also like the new find stuff, although I think it still needs work in areas.
Now on to the gripes
First, a quick bug. Classified Clicks all show 0 in my profile, but in normal viewer, are fine.
Now specific windlight stuff. The water sun and moon reflections are horrible. Completely unrealistic TOO WHITE. And in general, there's a little too much of it. THe color shouldn't be SO white. It also becomes completely non-transparent. It just needs work.
Second, as everyone else has mentioned, the light makes avatars look kinds crappy. I understand having the lighting effects on avatars with natural light, but they're just a bit too dramatic.
ON a regular basis, I use SL across both my monitors, and in the normal viewer, the performance is better than the windlight viewer. Everything else seems pretty good. Frame rate is a tad slower in some areas, but for example, in my workshop in the sky above my store, it's much faster than with the regular viewer.