Liquid latex tends to go slightly brittle after a couple years, depending on your environment. It's nice in that it's really flexible. You either want to use it on a porous foam or spray all sides of the prop (creating a complete shell) or it might not stick that well. They sorta skipped the fact that it takes something like 5-6 coats to get good durable coverage. Liquid latex tends to dry tacky, so once the final coat dries, you need to powder it with something like talcum powder or cabosil.
Also, you need to be careful when using it with sprayguns. In most spraygun models, if you don't clean the gun out completely before the latex dries, it is a huge pain to completely disassemble it and clean all the dried latex out. After spraying, you switch it for a bottle of water and spray until it runs clean.
Also, you don't need to use true liquid latex. For most purposes, latex-based housepaint works nearly as well, plus it's easier to get ahold of. Latex is only needed when it needs a lot of flex.
Here's another great vid demonstrating its use: http://www.youtube.com/watch?v=5Q1DPtL6iwU