I've been using Opal 1.2.2 for the last few days.
I find this program very comfortable to use - it is easy to make it fit my needs and the way I work. For instance, I can easily enlarge the size of the text on my screen without changing font size. I can give alternating lines a different colour of my choice to easily differentiate and enclose lines of text.
It is obvious that the creators of Opal have put a lot of thought and experience into making this program. They seem to know what a user like myself needs.
I had occasion to contact the programers - on a Sunday! - and was quite astonished to have almost immediate responses that were both succinct and helpful.
The help section is excellent. One thing that puzzled me, though, was that the part about preferences says that you can set a preference to allow you to give any menu item whatever keyboard combination you want. I couldn't find this in the preferences but would love to have it in the future. Or maybe someone could tell me how to do this in case I have just missed something.
Another thing I would like to see would be a highlighter feature with a button on the toolbar - such as in Scrivener or DevonThink, among others. There is a workaround possible in Opal by using copy and past style, but this isn't quite as convenient as a dedicated highlighter.
Personally, I am very grateful for Opal and look forward to using it for some intensive and constructive outlining - as I used to do with ACTA.