I've been using nvAlt for a while. Very fast search and ease of use. Not sure how well it scales in the long run but thus far I've had no problems. Edit to add

I've been using nvAlt for a while. Very fast search and ease of use. Not sure how well it scales in the long run but thus far I've had no problems. Edit to add: Some other interesting features. You can sync to Simplenote automatically, and notes can be saved as text files for easy import/export. There is some markdown support (though I see a number of bugs in the preview). The main downsides appear to be: limited color schemes, no syntax highlighting for markdown, development appears to have stalled.