savethytrees (12/14/2011)
In the missing index report there is a column called 'user_seeks' from the view sys.dm_db_missing_index_group_stats.I have seen significant improvement only if the number of user_seeks is also high. Check that column to ensure that the index being recommended will have higher number of user_seeks.
As previously suggested I do not create those indexes that might have high % improvement but low user_seeks.
This one case is more about index size than usefulness. It's one thing to improve the reads, another one to cripple the writes (including backups, maintenance, restores...) in the process!