The "as" operator as described in MSDN is an operator: "Used to perform conversions between compatible reference types". We have to keep this in mind when coding. Often it is just easier (and it looks nicer too) to use an "as" operator instead of a cast, or when we want to handle the DBNull value returned from the database like in the following scenario:
string s = reader["Name"] as string;
string s = (string)((reader["Name"] is DBnull) ? null : reader["Name"]);
Assuming that the name column can be null in the database.
When we use it we accept that something could be of incompatible type such as Name column being of numeric type return. What happens then? What happens if the column type has changed? In the scenario above, we will always get a null reference, which in a worst case scenario we will discover very late in project.
Of course in some situations it is just what we need. What I have found however is that misusing the "as" operator can often lead to unexpected NullReferrenceException exceptions being thrown (or worse yet nothing wrong happens at first). If you find that happening it may mean that you are using "as" operator as a shortcut where an ordinary cast would be in place. Using a cast often causes bugs to appear sooner in the development life cycle which is always a nice thing. So basically a rule of thumb is to always think twice whether to use a cast or an "as" operator.