I also agree, even though Fellers uses Bleeding Cowboy all over the place, their catalog is one of my first gotos when looking up a material, and I've probably read through it multiple times while on the crapper!
As others have mentioned, some of the major plus's:
Pics and good thorough descriptions of the material
An unpriced as well as a priced catalog
Good indexing as well as categorical (although sometimes things end up in more than one category so best to just hit the index in that case)
One of the things I really like is they give a rough Pantone match on their vinyls. Probably doesn't pertain to what you're looking to develop, but gives you an idea of going above and beyond the typical information a catalog has.
Catalog being available to download as well is nice, and they require you to have your customer id/etc to get the priced version to prevent outsiders from the industry getting the pricing info.