PEX is a type of Polyethylene. The advantage of this would be the cost per foot for the tubing, however that is about the only advantage in using it for going between brew kettles.
PEX/PE is best suited to be set and forget. It is not very flexible and continuous moving like during the brew process can cause leaks in the connections. Another thing about it is that it starts to soften up at higher temps. Then when it cools down, it becomes rigid again. Not good for your hoses. Personally, I would stick with Silicone or other standard hose materials.
Where it can be good is in kegerators and beer lines to faucets. There are some products which are designed exactly for this purpose. Bev-Seal is one of these. I recently bought some of this to use in my Keggerator. I was expecting it to be like any other beer line, but was a bit suprised when I received what is basically PET. The nice thing about Bev-Seal is that it is glass lined so that no flavors can leach into the PET or leach any flavors out of the PET. It work REALLY well and using it with 1/4" fittings does not require any crimping or clamps. One thing that you do need to do however is use a heat gun to heat the tubing just enough to get the barbed fitting in. After its in and the tubing cools, it shrinks down to form a very tight seal.