MrExcel.com has a rating of 4 stars from 1 review, indicating that most customers are generally satisfied with their purchases. MrExcel.com ranks 14th among How To sites.
I have learned a lot from this site, but mostly from the forums. The only downside is you might not get an answer if your question isn't phrased well. There are other useful articles on the main index, but some material is outdated.