I just finished reading FSoG.
When I look around the internet and read the discussions on it (as well as hearing feedback from a couple of readers), it seems the consensus is that the writing is bad.
It looks like I'm in the minority on this, but I don't think it's that bad, writing-wise. I've certainly read worse published novels. I had pretty low expectations before starting, so that might've influenced my reaction.
Regarding the writing, my only complaint is that she's pretty repetitive with her incessant use of "holy crap" (and other permutations of the phrase). Sometimes, she repeats similar descriptions too often (e.g. raising eyebrows). Other than that, she's okay. I can tell she's educated and has a fair command of the language. Could it be edited better? Sure. But horrible writing? Not in my opinion.
That said, my take on what's good writing may be pretty out of tune with the literary world. I read a book that was a National Book Award finalist last year, and I hated it. Thought it was by far the worst writing I had read amongst the 50ish books I read the last couple years. The author would have these long, overly flowery sentences that run on forever (one time I counted at least four long clauses). The pretentiousness almost drove me to puree my eyeballs in a blender.
The story is probably the part that fails me, but to be fair to E.L. James, this is just my own taste. There is too much sex, and that gets monotonous after a while. I found myself skimming through the later sex scenes. But I guess if you're reading it for the sex, then it fulfills that duty. Not much of a story arc - pretty flat there. Although I suppose there are also books 2 and 3, so maybe the plot develops more later. Book 1 is really just about the two MCs' relationship (in particular the sexual). I'm intrigued by Christian's backstory (how he acquired his sexual preferences), but probably not enough to read more.