I can't recall any times that I've had any problems running Sibelius 6. I currently run it on a Mac Book Pro that has a 2.2 Intel Core 2 Duo processor and 4 GB of RAM, and on this machine it runs pretty flawlessly. While it really depends on your system, Sibelius 6 doesn't take up too much processing power and I would imagine that those with less sufficient machines than the one I previously mentioned shouldn't have too many issues either, although it is hard to say until you try it yourself. I have always been able to run as many MIDI instruments with notation as I'd like and have done as many as twenty or more at once at times.
I've been using Sibelius 6 for only a short while, but I was familiar with earlier versions on Sibelius so it made the transition to this software quite easy. There are some cool extra features in Sibelius 6 when compared to some of the other versions, but not enough that I would suggest upgrading if you are already running Sibelius 5 unless you use this program everyday. Personally, I only use this software every once in a while because I don't deal too often with music notation. Sibelius 6 is pretty much as good as it gets when it comes to music notation software and I would highly recommend it to anyone looking for a great piece of music notation software.