I had 35% on sides and rear of my S60 T5. On my R I now have 30% front sides, and 20% rear sides and rear.
Both cars you can see out just fine at night. The auto-dimming rear view mirror also dims a little less to compensate (as you would expect).
I have 50 percent on my fronts because that's the darkest we are allowed by law in Ohio, and you can barely notice it.
On the rears I have 30 percent and its perfect. (In Ohio the rear can be tinted as dark as you like)
20% in daylight. This is the darkest I'll get for any future car I own as well. Its not to dark to the point that it looks tacky and its not to light where in my opinion theres no point in having tint.
Depends on the color of your car. My 05 t5 is white and I went with 15% all around. It's really dark but I don't have a problem seeing out. Lighter cars look great with heavy tint and it really brings out the lines of the body. Looks aggressive and in my opinion gives great curb appeal. I have 18" black mk111 by mkw with machine finish and it goes great with the black tint. Any dark color besides black I would go a bit lighter but nothing less than 25%.