Just so people know. I am now giving this contraption away, to the first person that can come and collect it.. See details on https://www.facebook.com/floweringelbow/
Lets evaluate the DS in relation to the design goals which were:
1) Effective removal of dust from hand-held tools and bandsaw
2) Little or no noise
3) Provide strong but wheelable work surface
1) The removal of dust thus far is excellent. The dust is sucked up and separated by the cyclones into the collection barrels. Because the separation efficiency is so good suction remains very high - no regular cleaning filters or changing of bags required. Obviously the collection barrels need emptying occasionally, but being many times bigger than a bag or standard shopvac canister, this is an easy and infrequent chore. Thus far, I have only had to change a vacuum bag after I got carried away and let the barrel become full, which resulted in the dust quickly clogging up the vacuum bag and suction becoming very weak.
So yeah, it might be worth me trying to make some kind of warning sensor that tells me when the collection barrels are approaching fullness to avoid similar problems in future. I already put a viewing window into one of the collection barrels, problem is that the static causes it to be obscured with dust, so that's little to no help. Some of you have already made some good suggestions on how to overcome this little problem in the comments. Of course any other ideas are very welcome...
2) Noise wise I am better pleased that I expected to be. When the DS is all closed, shut up and operational, I can't really hear the noise of the vacuums at all! The noise of the air rushing through the hose is pretty much all that is audible. So jackpot on the sound front. I can't tell you how nice it is to be able to clear up the shop and suck up dust without a loud noise. It makes nice quiet, well balanced power tools more worthwhile ;)
Now bearing in mind there are many differing and complicated techniques of sound measurement, the audiophiles may want to look away now. In a blissfully and probably horrifyingly simplistic manner, I used a mobile phone with an in-built 'sound meter' to do my measuring.
Sound of both vacs out in the open - 85dB
Sound of one vacuum in the open - 83dB
Sound of both in the DS - 61 (but varies a lot depending on where the end of the hose is situated - the air rushing in at the tip is almost the only precipitable noise)
Sound of one in the DS - 55
3) The work surface is nice, functional, and sturdy enough to dance on. I do need to add a breaking mechanism to the wheels, so that I can lock it in place better.
The DS has been a long project for me, with plenty of help, research and tweaks needed along the way. Still, it has come together in the end and with any luck this instructable will help you guys avoid some the mistakes I made. Already a number of you have said you will be making your own DS, so I look forward to feedback, build photos, and areas of development. If it improves the working environment (and health!) of one of you, my fellow makers, hackers and craftspeople, then great!