String.IsNullOrEmpty should not be used in production code, as it currently has a bug.
http://msmvps.com/blogs/bill/archive/2006/04/04/89234.aspx
Also, IsNullOrEmpty is redundant in VB. You get the same results if you just compare a string with an empty string.
Dim s1, s2 As String ' Default value is empty string
s1 = Nothing ' Explicitly set to Nothing
s2 = "" ' Explicitly set to an empty string
Console.WriteLine(s1 = "") 'true
Console.WriteLine(s2 = "") 'true
Console.WriteLine(s1 Is Nothing) 'true
Console.WriteLine(s2 Is Nothing) 'false
Console.ReadLine()