Using DTA vs. evaluating DMVs?
DTA helps for adding indexes but not removing indexes.  If you're faced with a database that already has more than, say, 5 indexes per table, the DTA isn't the answer.  You'll want to use the DMVs to evaluate usage of existing indexes.

Here's a script with a video tutorial I did a while back:

http://www.toadworld.com/platforms/sql-server/w/wiki/10062.find-indexes-not-in-use.aspx