Kushiel's Dart

and the series that followed: intriguing fantasy drama or mediocre BDSM smut?

Both.

And her "Sundering" series: metafictional commentary on high fantaasy or crappy Tolkien fanfic?

I don't feel entirely qualified to answer this since I've only read the first book and they were meant to be one book only, but both in this case as well.

I see what she tried to do there but the result fell short. The  characters are flat and uninteresting, the plotting is nothing to write home about and the bad guys are shafted throughout the story (whether or not it is intentional I think irrelevant).

I'll still read the sequel due to the aforementioned reason though.

Still, it is rather interesting for a series which has a premise which can be roughly summarised as, "Phedre gets the crap beaten out of her and LIKES it".

The whole "French people are sissy degenerates" is quite funny too.