Every couple of months I'm in a meeting where a couple of developers start arguing about which HTTP status codes to use in their RESTful API, or where they decide to not use HTTP status codes at all and instead layer their own error-code system on top of HTTP.
In my experience, HTTP status codes are more than adequate for communicating from servers to clients. Furthermore, it's preferable to stick with this standard, because that's what most client and server-side HTTP libraries are used to dealing with.
When it comes to which status code to use, the truth is that most of the time it doesn't matter, just so long as it falls within the correct range. In this post I'm going to outline what the important ranges are, and when you should use each one.
If you control both the client and server, these guidelines should do just fine. If you're writing a more generic RESTful service where other people are writing the clients, you may have to be a bit more nuanced. Either way, this rule-of-thumb is a good starting point to work towards the simplest solution possible for your particular problem.