Intellisense in SQL Server 2008 is a time saving feature. But sometimes while writing queries, it starts underlining some object names or some column names which are already added in the database and these are correct but intellisense marks them as a MISTAKE.
In long queries, it creates confusion. I have seen my fellow DBAs turning off the feature to avoid confusion and mental stress;).
Also, sometimes intellisense is not helpful enough for auto-complete feature.
A simple solution for this problem is to refresh local cache by using Ctrl + Shift + R. So it should start marking only real mistakes. You can also refresh local cache from Edit--- >IntelliSense --- > Refresh Local Cache.