Well, to each his own, I guess. But Heinlein was the first author to but a book at #1 on the NYT Bestsellers List. For a time, he was the only sci-fi author who could write bestsellers. The Cat Who Walks Through Walls and To Sail Beyond the Sunset, among others, also reached #1 on the NYT list.
I suppose every book published received at least a few bad reviews, as did Stranger in a Strange Land. But I wouldn't say it received mixed reviews. The reviews were overwhelmingly positive.
Having said all that, Heinlein is not for everyone. He tends to write socio-political treatises disguised as science fiction.