Fifty Shades is good erotica because it met the needs of its target audience and they greatly enjoyed it. From that perspective I do in fact consider it well written. It was written such that the target audience enjoyed it--had it been written in a more "literary" way, many of them probably would have enjoyed it less. or not at all.
IMHO, one of the common traits of erotica writers and aficionados is to respect that people like different things. Some things are broadly popular, some things have a niche audience, some things are not good examples of their type or fail to find an audience.
Given that some people think the entire genre is not only bad but evil, it is best not to consider ones own tastes, not matter how highly refined, to be a basis for making absolute judgements about quality. I love me an angry whopper and consider foie gras despicable in every basis including taste, but there is not a Michelin star chef in the country likely to agree with me on that, and it really doesn't matter. I am not their audience.