Well I'm glad to see a Cambridge SoundWorks thread on here.
I am currently using MC300s across the front, S300s for surrounds, and a P500 and P200 combo for the bass. This is a setup that I have found really enjoyable, especially for movie watching. But it's great for music too. One thing that I have been impressed with is the imaging that the fronts produce. I have several 3 channel SACDs that give you a distinct "you are there" feeling. My amplification is provided by a Marantz SR5500 that is not overly powerful at 90W per channel, but I don't have any trouble filling the room with sound with these speakers. Dialog is very crisp as well.
I originally bought the system along with the P500 sub. This is a great sub for music I think, especially electronica where the demands on the bass are very high. It sounds tight and accurate. However, I felt that I was wanting in a couple of areas. First, I didn't feel that I had the low end extension and volume that I wanted when watching movies. Second, I have a fairly large listening area, and I wanted more even bass coverage. From some online reviews, and the fact that the P200 has a 10" driver vs the P500's 8 inchers, I thought that it could help out with the low end so I gave it a try (I didn't want to spring for a P1000 though). My initial reaction to the P200 was a bit underwhelmed, at least for music listening. The sub is deceptively powerful though. Once it was broke in, it packed a punch, and it was really impressive in movie watching. I have the P200 running with a low crossover now, and the P500 focused on the higher bass frequencies, and the improvement has been tremendous. It feels like I have more than double the bass. It also sounds a lot more even throughout the room. This setup can really rattle stuff and actually knocked a fairly large mirror off the wall. Oops!
I'm planning on using this setup for awhile, but down the road I could see myself doing some upgrades to improve music listening. I think that a pair of the passive towers (either the T100s or T200s) and an MC400 center channel would improve musical performance through the mid-range. The T200 would probably require an amplifier upgrade though! But, I'm quite happy with my current setup, and there's nothing compelling me to run out and upgrade right now.