Lightroom 4 Lens Profile Correction

When I enable it, it seems to decrease the distortion and vignetting my Sigma 30mm has. But i'm not sure if thats all it's doing, so I'm wondering how do you tell if the image is better with or without profile correction? Is it just based off how it looks to my eye?

What YOU like your work to look like is the correct setting! I find most of my stuff looks better with the correction.. more even lighting and distortion. Do a before/after side by side and pick your fav.

I think a lot will depend on the lens as well. Some show more of a difference than others. I tend to like it on for the most part.

If your image has straight vertical or horizontal lines close to the edges of the frame you can flip over to the Manual section where mousing over any of the sliders will cause a grid overlay to appear. Maybe this can be done by mousing over the sliders on the Profile section, I'm not at home to check it. With the grid you can check the quality of the correction and tweak it if needed. The same sliders on the Profile section can be used to apply only partial corrections if you like the effect better.
Regarding vignetting, it's kind of funny that I remove it with the lens correction and then (in landscapes, etc.) always add at least a little back.

I find that I use the Manual mode more often that the Auto mode.

To confirm the performance of the correction profile, take a shot of a grid - could be a checkerboard like the pattern you use to generate a custom profile, could be an office building with lots of windows in a grid, etc. Then you can apply the correction profile and see what it is actually doing. Try to minimize the tilt and pan of the camera relative to the surface (that is, shoot as perpendicular to the surface as possible - the profile does not correct perspective distortion).

While "what looks good to you" may be okay, if you want to find out what the profile is doing, you should use some semblance of an objective evaluation to see what's what.

Have fun!

kirk