I use 4000 IU of NOW brand, daily.
http://www.iherb.com/ProductDetails.aspx?pid=8229&at=0

I think the liquigels are a better formula. I don't trust the dry
for being absorbed.

I was having alot of seasonal depression in winter.
The first season 2 winters ago I was only using 1000 IU, and I had to buy a light visor for morning use.

Last winter I went up to 2000IU and used the visor about 1/2 less.

This winter was 4000IU and I used the visor for only two months and at that twice a week or so. Just didn't need it.

I don't take it in summer when I am outside and tan up alot.
I always feel better in summer!

I have not had it tested, but I can feel the difference very clearly between the doses I used for these 3 winters!

You know there is a cholesterol analog in the skin that makes Vit D. So I think people who use statins and get their cholesterol really low, then become Vit D deficient from not being able to make it in the skin anymore. Also I have read that aging reduces this conversion anyway!
There are studies now showing depression linked with low Vit D levels.