It's pretty secure (although probably not FBI-proof). The biggest problem comes however if the key is stored in plain sight somewhere. I think that defining it in the property pane will mean it is stored in plain text in the EXE (or at best compressed text).
So the way around it is either to always ask the user to choose keys, or to come up with the key in another way (perhaps take the users name, reverse it and hash it and use that as a key or something).
I'm glad you've had no problems DizzyDoo and you are enjoying it byo.
Remember to post suggestions!