Mike's comments are similar to my own experience. Although Sennelier's are my favorites in color range, some are harder and some are as buttery as can be. Certain colors cannot be achieved in any other color palette which is good. The downside is that some are very hard, nearly unusable, but those I do not use much anyway and can find in other mfgs.
It also depends on your surface. If you are using Canson paper Sennelier is better used last, after NuPastel and Rembrant layers as a final soft application.
If however, you are using a very "toothy" paper or board, they are wonderful. It used to be that Sennelier was the only soft, buttery pastel available. Now there are many. But whatever soft pastel you use, as Mike said, you have to try/test to see what works best for you, your subject and your style.
If you have an art store nearby or can order a few open stock or small set of Sennelier, I encourage you to at least try them. Then, if you use a full palette of colors/values for your painting, purchase a full set, replacing only the ones you use most often after that.
L. Diane Johnson
Plein Air Workshops