How to keep an audit/history of changes to the table

I was recently faced with a requirement to audit some tables and I opted to use triggers. Like others, I only wanted to see entries in the audit table for those fields that had actually changed, however, when updating the tables, the application was updating all the fields in row whether they'd changed or not, therefore, checking whether the fields had been updated or not availed me nothing - they all had!

What I wanted, therefore, was a method of checking the actual value in each field to see if it had changed or not and only writing it to the audit table if it had. Having been unable to find any solution to this conundrum anywhere, I came up with my own, as follows:

CREATE TRIGGER [dbo].[MyTable_CREATE_AUDIT]
ON [dbo].[MyTable]
AFTER UPDATE

AS

INSERT INTO MyTable_Audit 
(ItemID,LastModifiedBy,LastModifiedDate,field1,field2,field3,
field4,field5,AuditDate)
SELECT i.ItemID,i.LastModifiedBy,i.LastModifiedDate,

field1 = 
  CASE i.field1
    WHEN d.field1 THEN NULL
    ELSE i.field1
  END,

field2 = 
  CASE i.field2
    WHEN d.field2 THEN NULL
    ELSE i.field2
  END,

field3 = 
  CASE i.field3
    WHEN d.field3 THEN NULL
    ELSE i.field3
  END,

field4 = 
  CASE i.field4
    WHEN d.field4 THEN NULL
    ELSE i.field4
  END,  

field5 = 
  CASE i.field5
    WHEN d.field5 THEN NULL
    ELSE i.field5
  END,

GETDATE()

FROM inserted i
INNER JOIN deleted d
ON i.ItemID = d.ItemID

As you can see, I'm comparing the values of each field in the deleted and inserted tables and only writing the field value from the inserted table to the audit table if they differ, otherwise I just write NULL.

It certainly works for me. Can anyone see any issues with this approach? My team own both the application and the database so possible curved balls like schema changes are covered off.