The Any method can cause the enumeration of the IEnumerable, if it doesn't have another way to determine the result. In some cases it can be a problem, for instance if the IEnumerable instance is actually an IQueryable that will cause a database query or web service call to be executed. Now, if it's just an in-memory collection, it's not really an issue, because enumerating the collection won't have noticeable side effects. And anyway, Any will use the Count property if the sequence implements ICollection, so it won't cause an enumeration.
