If you have a poor performing keyword does it have any affect on any of the other keywords in the campaign? The keyword in question still gets clicks which may produce sales so happy to keep it there but the CTR for that keyword is very low in comparison to the other keywords. My own concern is that it brings down the overall CTR for that AD group - does it matter?

That is hard to know for sure, stickyboy. Google Reps tell me that the keywords are judged individually and so are the ad groups, campaigns, and the entire account. How each of these factors is plugged into any of Google's many algorithms is anyone's guess.

Google Reps tell me that CTR is calculated individually for each keyword phrase and other phrases do not affect the CTR of that phrase.

Even though I've had Google Reps say it isn't necessary, the most knowledgeable PPC consultant I know feels that they do affect your account and still advises not running good and bad keywords in the same ad group.

Nothing brings down overall CTR in an ad group more than running content ads and that doesn't seem to matter so the best I can tell you is it probably doesn't matter but you might still want to put your most important and strongest keywords in their own ad groups.

[NOTE: I run very few content ads and have content turned off in almost every account I manage. It is usually the first thing I do when I get a new client.]