Alex,
The gel spun ‘super’ lines have some advantages and some disadvantages, as Mr. Castwell pointed out.
They are thinner for a given test rating, so you can get more on the reel. Being thinner, they do ‘cut’ the water better. They are ‘slicker’ so they travel through the guides with less resistance.
Gel spun lines are resistant to UV light and last far longer than other fishing lines. They don’t absorb water, are temperature stable, and are bouyant.
The ‘slickness’ of the material can cause some problems for us as fly fishermen. Knotting them takes care. Knots are prone to slippage and many ‘fishing’ type knots are not suited to the gel spun lines.
If they are not tightly spooled, the slickness of the line can cause it to ‘bury’ into itself on the spool. This can cause a ‘jam’ on the spool, locking up the line and causing a break in the tippet or the backing when a fish makes a strong run. Few fly fishermen pay close attention to tightly spooling their backing (the 1 to 1 gear ratio of most fly reels makes this more difficult than conventional reels).
Gel spun lines have near zero stretch. Combine this with their inherent ‘slickness’ and you have a serious skin slicing issue. Never grab and pull on these lines with your bare hands. Never grab onto one of these lines when it’s tight.
Power Pro is an unusual gel spun line, in that it has been built to be cylindrical in crossection, and won’t ‘flatten out’ like the earlier braids (spiderwire, gorilla braid, the kite strings). This helps greatly with the burying problems on the spool, but does sacrifice some capacity due to a bit larger diameter.
Up to you if you want to use it. Many folks do use some kind of gel spun line for backing, just about all the major fly fishing suppliers that manufacturer a backing also make a gel spun version.
The primary reason for most of the folks that use it is the increased amount of backing they can get on the spools. If you fish for long running fish where you may need an extra 100 yards or so, then it may be for you.
Just helps to know the pros and cons in advance.
Good Luck!
Buddy