Just to add one more answer to the pile, there is a System.Runtime.Remoting.Metadata.W3cXsd2001.SoapHexBinary class that I've used which can convert bytes to and from hex:
string hex = new SoapHexBinary(bytes).ToString();
byte[] bytes = SoapHexBinary.Parse(hex).Value;
Not sure how it compares (benchmark) to other implementations, but IMO it is pretty simple -- especially for converting from hex back into bytes.
