stu, here are my thoughts:
I currently run three MC500's up front. I started with a MC300 center channel but found that it just didn't get me the authority up front that I wanted from a center. It wasn't a bad speaker at all, but I play movies at very high volumes and it simply couldn't keep up. The MC500 is an awesome speaker, and getting two more to use as my front mains was a great decision.
I think that anyone that watches movies at moderate volumes would be ok with MC300's (my Uncle has three MC300's up front) but if you like to really crank things from time to time I'd recommend something a little bigger.
My father runs four S300's for his surrounds/backs and they sound incredible. I look forward to upgrading to those around the holidays this year.
Overall IMO Cambridge Soundworks makes great speakers for the price, but I will be the one to go against the grain here in this thread with regard to their subwoofers.
I have NEVER been a fan of Cambridge Soundworks subwoofers and believe that they are VERY overpriced for what you get. Years ago one of the first subwoofers I purchased was the Basscube 12 which at the time I believe was their biggest/best sub (before the P500 and P1000 models). Of course being one of my first subs I thought it was great.
Anyway I went through subwoofers like water for about 5 years or so. During that time my father (after he scored the S300's) dropped $999 for a brand new P1000. He figured that since he was very happy with Cambridge Soundworks quality so far, why not go for the P1000. And for $999, it "has" to be good right?
I was never before so disappointed with a subwoofer in my life. The P1000 to me sounded incredibly lifeless; it had NO authority and didn't play low at all. I mean just looking at the thing left me scratching my head. I don't recall the exact dimensions, but it's got to be about a 14" cube (picturing it and its two 10" drivers) and the thing was so light I could pick it up with one arm. Sorry guys but a $999 subwoofer should be fairly big, certainly heavy and should perform very well. The P1000 in my opinion performed exactly the way it looked - small.
I own two low end subs that are currently sitting in a spare room in my house... a Cerwin Vega CLS15 and an MTX Sub 15 (both 15" drivers obviously) both have about 250-300 watt amps and both play louder and lower than the P1000. Both 15" subs I purchased for about $275 each.
My father ditched the P1000 (actually sold it to my Uncle who runs the three MC300's up front) and he went all out on the SVS PB12-Ultra+2 and I currently run a PB12+2. The PB12+2 does circles around the P1000... louder, lower, more accurate, the list goes on.
This is just my personal experience with the P1000... for those that have posted that they like the P1000 have you swapped it out for a PB12+2? It's only 20% more in price but EASILY 100% more (twice) performance/sound. Again, this is coming from a guy that loves Cambridge Soundworks speakers (non-subs) and believes they make great bang for the buck products.
Another thing, I highly recommend ebay for Cambridge Soundworks products. While the hifi seller of b-stock items from Cambridge Soundworks offers decent deals, I've found other sellers that crush their prices.
The three MC500's that I've purchased in ebay I got for $169, $156 and $127. Two out of those three were brand new/sealed and the third was slightly used at 2 months old. The hifi.com guy sells them for $211 buy it now. Still a solid deal over the $399 new from site price, but if you watch auctions for a week or two you can find steals.