Yes, I do! I change it at the end of every month. I've also just discovered that you can use words for iPhones, so I will be implementing that in about 3 days' time :) Quite excited for that.
I think it's necessary to have a password because people these days are absolutely ruthless when trying to find the goss, or sneaking through the contents of other's phone - I will never understand what is the benefit from such behaviour. It feels like a huge invasion of privacy.
Yes, I do have a password. I think it may have been the reason that I was able to retrieve my mobile when I lost it a few weeks ago. I simply rang my number and spoke to the finder. It was no use to him with a password on it. Giving him the benefit of the doubt I gave him a cash reward. My husband has since put a password on his.
Not for my phone itself, but I do have certain apps password protected. Each one that has this has a different password or code, so I think that makes it a little more secure than just one password and then you can access the whole phone.