RowGuids (uniqueidentifier) are large fields (16 bytes) and are basically going to ALWAYS be unique.
SQL Server adds a RowGUID column to all tables if you are using Merge Replication (but doesn't add an index).
RowGuids in general slow things down. Some people may consider using a RowGuid as their primary key. This is a bad idea because the index is going to be quite slow.... you are searching a large field. It goes without saying, NEVER have clustered index on a RowGuid.
Another little annoyance I find with RowGuids is when you are searching for one. You can't use > or < on a RowGuid
Note: There are not many cases where a RowGuid should have an index on it. (Exception SSW SQL Total Compare which is a tool that compares data is in sync via rowguids and this makes it lots faster).