It has been debated many times on various forums.
I saw an article (linked on ZHPMafia I think) in which they measured the distance between the strut towers and concluded the displacement was only a millimeter or two. IIRC, it was also done on a track, where stresses are usually higher than street-driven cars.
Conversely; Those German engineers are pretty damn smart, and if BMW added it to the M3, it must do something.
I would have added one but haven't because;
1) I can't find a good one which is reasonably priced and meets all of my needs
2) Most are fixed and therefore cannot accommodate camber adjustments without modification.
3) Most are bent, and as any structural engineer will tell you, if it is bent it provides less rigidity than something straight.
-Just my two cents.
(
Edit) Found the
link, and correction; the delta was actually only
0.7mm at the highest stress point.