I quite like NLS. I was aware of what it does before I bought it, it simply models the characteristics of channels on certain mixers with the convenience of being able to alter a bunch of hem at the same time and using SSL for some tracks, Never for others and so on. I use it with Logic, in Sonar I'd probably just use the PC equivalent.
Like most such processors it's pretty mild in what it does, which isn't surprising given that the desks it models were designed and built with low noise and low distortion as a priority. Using an analogue mixer for summing really does pretty much the same thing - a set of channels each with it's own characteristics and minor differences which are then fed into a master bus which itself has some "imperfections".
As a way of making stuff sound less pristine and "digital" it does it's job, though on a project with few channels it makes little audible difference - which is how it should be, if it had a pronounced effect on audio it wouldn't be acting like a high-end desk does.
As for using it with things like Waves REDD or TG12345, I wouldn't go down that route as those plugins model very different kinds of desks to the more modern ones and have their own characteristic sound "built in". Unless you've a use for emulating a setup where a 50s or 60s console is then plugged into a more modern desk.