Anything that is suitable for plain text gets stored in nvALT/Simplenote rather than Evernote. For me, the speed and simplicity of plain text is important. I have notes with everything from lists, frequent flyer numbers, and keyboard shortcuts I commonly forget.
I have decided that the convenience of using just one app everywhere outweighs the speed of plain text notes. I now use Evernote for everything. For quick entry on OS X, the menu bar icon with a keyboard shortcut comes in handy. On iOS, I use Drafts to quickly capture text. Drafts then allows me to send the text to Evernote. I’ve even set up some custom actions that allow me to append text to certain notes.
As for tags, I use them in Evernote for ease of finding documents.
For simplicity I’ve done away with tags. With the excellent search and OCR capabilities of Evernote, I find that between the titles and content of all my notes/documents, I have no trouble finding what I need.
I’ve found offline notebooks (Evernote Premium feature) incredibly useful with this change. I made my notebook for my quick notes (what I used to keep in plain text) offline so I have easy access to it on my iPhone if I don’t have an internet connection.