I quite often get request to summarized my ideas about Removing bookmark lookup on this blog post. Bookmark lookup or key lookup are bad for any query as they force query engine to lookpup corresponding row in the table or index as it does not find required data from just reading the data.
Here are list of my four post written on the same subject.
Let me know your opinion about the same and I am interested to here what you have to add to this subject.
Reference: Pinal Dave (http://blog.sqlauthority.com)