I will answer without reading the rest of the posts, so hopefully won't duplicate too much.
My Setups
Baitcasters - I use either 30# Suffix 832 or 50# Power Pro on all but one that I have been trying Yo Zuri Hybird on, don't like the lack of feel but otherwise it is ok line. For baits like swim jigs I will use 20# sniper FC leader tied with a modified alberto knot which is 6-12 feet long.
Spinning - 10# or 20# fireline with 6-12 ft red label FC leader also tied with a modified alberto knot. Fireline has been my goto spinning setup since I came out. I tried other braids but have had zero reason to change. I know fireline is not technically a braid, but its sensitivity is right there with any braid I have ever fished. Great line, hope Berkley makes it forever!
Performance with old rods has been good for me. The only rod I ever had an issue with was one that had plastic line guides. What I have noticed with braid compared to mono or the yo zuri hybrid, is the sensitivity is increased exponentially. You will think that you upgraded rods with the switch. That is the biggest problem I have had when I tried straight Fluoro, I really missed the sensitivity of braid, especially for bottom contact baits.
Biggest tip with braid - Loosen the drag due to the zero stretch. I also like moderate action rods and feel they suit braid very well. Older rods generally seem to run more of a moderate actions, so they may work even better than the newer faster tip models.