I absolutely love most of NuSEF (used to be called Open-SEF).
The feature set is great! Very easy to use.
However, as my website got bigger (5000+ pages), my website took a long time to load when NuSEF was turned on.
NuSEF turned on = 4X longer to load than when NuSEF is turned off.
This is on a shared server at a very big Service Provider.
Memory usage increased above the standard 32MB limit allocated by shared servers to my account. I had to allocate 64MB via .htaccess file in order to get my page to load.
All of these problems went away when I turned NuSEF off.
So, reluctantly, I am switching to a different SEF component.
Switching your SEF on a mature website can be very costly, since some of your URLs will change, and that can cause you to lose a lot of previous SEO work. It can worsen your Search Engine page position and reduce your traffic. It is important to chose the right SEF component early in the life of your site.