You kidding me? BIONICLE was like the stuff. It was MY action figures, not GI shmoe or w/e. You could buy two different sets, build them, and mash a mix of both to make something supercool. Or buy two of one set and combine them to make a super-thing.
The possibilities with BIONICLE were endless.
It's because how it's not very useful for anything outside of the line, them by their own is pretty cool and the story background gives quite a bit of immersion.
But as you say "...and mash a mix" It's really just click arms and legs here or snap two bodies together to get a big guy. Mr.Potato head has more options and creativity, But then again I've not bought any Bionicle sets for around 10 years.
I've got a whole 8L tub of it, and it's nowhere near my LEGO collection ;D
tl;dr: It's ok, by itself. I just wished it was more compatible as anything LEGO should.
But because I don't have any of the new ones I think my objection will be overruled.