I'm in the process of building a 20x24 deck using Dek Blocks, and so far so good. I just finished putting up the joists this weekend and installed a few feet of decking.

This site has a lot of useful information, and I wanted to pass along one thing that made my life much easier: using a surveyor's tool, also called a transit, to level the blocks, along with shims cut from 4x4. My yard slopes about 3-4 inches from its highest point, and I wasn't crazy about using string to level over distances of 20-24 feet.

By using the tranist, I was able to identify the highest block and then shimmed all of the other blocks up to that height. The basic procedure was to make a drawing of the Dek Block locations, measure with the transit to determine how much to shim a block, write that down on the drawing, and continue measuring the rest of the blocks. After that, I cut the 4x4 shims and wrote a number on top that corresponded to the number that I wrote on the drawing, and then simply placed each shim in the correct block when done.

Sometimes my initial measurements would be slightly off after checking with a level, so I'd have to remove the 4x4 shim and shave a little off using a chop saw or cut a new piece if the shim was too short, but the initial measurements with the transit were always within 1/8" and usually right on.

A transit can also measure angles, so you can use it to make sure or verify that your deck is square.

Somebody tell me this is OVERKILL. If you follow the directions there is no need to try and level every pier to each other. I just finished a 12 x 24 wrap around deck around a oval 18' pool end. Never used anything but a level and a tape measure and the 1/2" gap around the pool's edge is spot on all around and I had 2-3" of fall. I don't think it need to be made any more complicated by adding a transit. As far as right angles go just use the Pythagorean theorem en.wikipedia.org/wiki/Pythagorean_theorem or if your not mathematically inclined use 3' 4' 5' or 6' 8' 10' to check squareness (where 3'4' or 6'8' are the legs and 5' or 10' is the hypotenuse).
Do follow the directions closely but no need to get OCD. Just my opinion.

I am in the process of building my deck (approx 750 sq ft) and I am using a water level to get all of my 4x4's at the same height. This ensures that each post is at teh same level.