I bought the whole collection not long ago and could not be happier with it! I've struggled for an art style for my game for years and feel I've hit it right on the head. It is different from the RTP, the color palette is more muted, which I like, and it feels like a game crafted with them can hold a more serious tone. As was stated by
@Anyone you can mix and match the sets (which I have done to create an amazing map just recently using all the FSM tilesets) recolor existing, etc. On top of all that, if you use the assets and edit/frankenstien or create your own content to match the tilesets, you can create unique pieces that are in your game only.
In a nutshell, I do not think it matters that others use the same DLC, as
@TWings mentioned how popular it is. It all depends on how you use it.