I read all three.
The series did not improve as it went along in my opinion. There are BDSM erotic romances that are much better (also many much worse). I think it took off because of the coverage due to the socialite book clubs that had adopted it.
Here's my take on this kind of phenom--if it increases the number of dollars spent in the industry, great. If it causes non-readers to become readers, even better. If it raises awareness of genres or lifestyles not otherwise in the spotlight, yay to that too. There's been news coverage to the effect that it has helped couples in their relationships. Hey, if all it takes is $9.99, some book inspired toys and some imagination to help people out, where's the harm in that?
For me, though, it was disappointing. Nothing new here, but maybe that's why it connected with so many readers--that and it's monogamous, single couple hetrosexual BDSM. If you like the movie 9 1/2 weeks, you will like the series.