I bought Missing Sync for iPhone from Mark/Space without reading the fine print carefully. I thought it would "sync" notes bi-directionally. The software actually only transfers notes on iPhone to Mac, but it does a good job.
Later on, I realized that this may not happen very soon even with 3rd party software. The difficulty of transferring notes on Mac to iPhone may be due to the fact that they are really different things. Mac Mail notes takes rich-text contents such as graphics etc. But iPhone notes are only plain text.
The software is expensive though ($40).