Vasko, I'm a big fan of Sunline castaway in the lighter line classes but haven't used it above 30lb. You need to bare in mind that the castaway is an IGFA rated braid which is designed to break at or below the stated breaking strain. So 80lb is intended to break at 70 - 80lb. As such you will notice that 80lb Sunline will appear thinner than (most) other 80lb braids - because they will generally break at higher breaking strains.
I've known some 80lb braids to break at up to 125lb. With that in mind, you might wish to consider 80lb Sunline against many other PE5 - PE6 braids to compare apples to apples. When making your decision, bare in mind Sunline Castaway IS designed to break! (at this strain) Many other lines are designed NOT to break for as long as possible.
Other than that, in the lighter line classes the light blue colour is bloody hard to see against the sky, and the water sometimes, and it goes a little furry after a while - I still reckon its a great line in the light lines - and my only pick up to 20lb!
