I love my molds! Although I second Mid's comment about time. It takes a lot of blocks and there's not as many as you might think on each mold.
Depending on what its for I use either Hydrocal - rock hard, readily available and can be purchased at my local model railroader store, or I use 'model' grade plaster of paris from Michaels.
The hydrocal is more expensive and definitely stronger, but heavier too. If I was to get more molds (very likely) I'd get the wall builder which is similar to the roof. Its a bit thinner than reg blocks but if its enclosed and you're not wanting to look inside its perfect.
Bruce's little vids and tutorials are great for helping to turn out perfect blocks.
I use a method similar to Mid's, except I make enough plaster to fill 2 molds in one go, wait 5 min for the water to rise and then do my leveling off, then I do a second batch. Then I either do another project for a bit or watch an hour long episode of something on the TV or more likely play "soccerball" with my 2yo son. Pop the blocks, clean them off, set them on some newspaper to help them dry and do the same process again. The second batch I usually leave overnight and just pop 'em before work. Typically I do this twice a week and at this rate its pretty slow.
I highly highly highly recommend starting small with your projects to get a handle on things and to give yourself some success. My first two were great, my third project is 1/3 done and it was started in September! Will look great when done however!
If at all possible buy larger amounts in one go, find a friend and then you can 'share' blocks when needed. I find I don't always need all the block on the mold but make them and then bin them for later. For my latest project I need on angle block that only is one per mold. I calculated I need 200 more to finish which is a lot of time! Plus if you buy more you save a bit on shipping and discounts etc.
DO NOT buy of eBay, for some strange reason, its considerably more unless you are buying someone's mold they no longer want. The dealers are buying from Bruce and then reselling. price is comparible, but S/H is outrageous usually.
Also, choose which line you want carefully. Not all the 'extra's go well together.
Have fun! its a great product but is by no means 'instant'. I thoroughly enjoy mine.