When candy companies mess with a classic treat, I get nervous. I'm not saying that changing a classic treat shouldn't be done, I'm just saying that often it's done very poorly. It's difficult because you have a certain expectation that comes with the original version of the treat, and you also want the new version to be creative as well. When it doesn’t work, I often find that it’s because the new version is rushed, sometimes candy companies don’t think things through, or research enough.
The folks at Mars decided in this case that the best way to change M&M's is to change the stuff in the middle. Mars has one advantage, they already know that using a crunchy textured center can work, since they've been doing it forever with M&M's peanut. What they chose to do in this case is pick a new center, something that creates a unique taste and texture, but still keeps it crunchy. This must have saved them a bit of time with the research and development of these particular M&Ms, since a big crunchy hurdle had already been crossed.
While the slightly salty taste of the pretzels is unique, the true wonderful change is in the new crunchy texture. It has a great crunch that isn't as solid as a peanut center, but instead, it's a cracker/pretzel center that breaks apart really well in the mouth. The texture of the pretzel center works well with both the candy coating and the chocolate layer in between.
Using a pretzel center is not a completely obvious choice, for the center of an M&M, but after eating it, it feels very natural. This makes it a great surprise, and that's always good for a new variety.