yes, it's strangely satisfying isn't it? I scraped mine into a jamjar so I could see how much there was (when it dries out there is much less of it)I was amazed at how much ruddy crud it caught in the first few hours alone.
are you running the CH pump round the rads, with the boiler turned down? it is worth closing them all except one to increase the flow through each rad in turn so the X400 washes out more of the sludge. open both valves but note the number of turns on the lockshield so you can put it back later.