About 3 months ago I followed the directions on this thread to seal an asym and a hex tarp I put together. I've only used them a handful of times in rainy weather so while it is too early to speak on how durable these tarps are but they are definitely waterproof. The only times I've gotten wet was while using the asym in a windy storm but that was only on the ends and I believe that was more of a function of my tarp not providing adequate coverage. The hex has been set up in 2 storms with high winds and heavy rain and performed beautifully.
Overall if youíre having to buy all the materials I would recommend just buying the silnylon from Scott at DIYGearSupply. He has great prices and Iíve been very satisfied with all the orders I've placed with him. You wonít have to spend that much more and the quality will definitely be higher. Now on the other hand if you already have some left over fabric from another project or already have silicone then it is a fun project and I do think itís a good learning experience. Now I feel comfortable applying silicone to my tarp and if I ever had to do repairs on a silnylon tarp I would feel more comfortable working with the materials.
I feel like this is the same debate that always goes on when considering going the DIY route or purchasing from the amazing cottage vendors that are present on this site. Overall their quality is top of the line and the prices are hard to beat. Now on the other hand assembling your own gear can be just as rewarding (and addicting) as actually using the gear!