In the Highpower Competition world where I play, N540 has a good following when used with the .223 Rem. We're typically getting 100 to 200 fps more with 80 gr bullets than the other popular powders such as Re15, Varget and N140 (of course with excellent accuracy).
I was concerned that the greater velocity was coming at the price of dangerous pressures. However the proponents were not blowing primers. In fact their primers were normal in appearance all the way up the ladder (I'll stop short of quoting 80's velocities to avoid the skepticism that's sure to follow). I was sufficiently baffled that I ran some incrementally increasing charges through my strain gauge equipped AR Upper. The pressures I obtained were consistent with the primer appearance.
I've been told that the 500 series gives more velocity by Prolonging the Peak Pressures and thus increasing the AUC of the Time/Pressure curve.
One downside that was highlighted by a friend that was in charge of loading for one of the Military teams was that in their experience, double based powders such as the VV 500's were more temp sensitive.
I am using N540 in a 223 with 75 gr. AMax @ 2915 fps and no excessive pressure at all. It performs very well at 500 to 600 yards. Actually N540 is the powder I use the most in sevearl rifles. The 260 with 108 gr. Scenars, the 308 with the 155 Palma bullet.
N550 works great in my 300 WSM with both 155 and the 175 MK's. And I have just bought some N560 to try in the 260 with the 142 MK's. I really like the N500 series of powders.