After I installed a Katzeye screen in my D200, I compared the accuracy of the green focus light to the Katzeye using a Zeiss ZF 85/1.4. I got ok results at f2-f2.8 with the green light, but it didn't cut it at f1.4. The Katzeye, on the other hand, was a revelation!
If you like manual focusing, I really recommend getting one. I got the standard non-brite treated screen and it works great. I like it so much I sold my 18-200mm and now use my D200 almost exclusively with MF lenses.