I own the Rio and the SA and like them both. However I must admit that I don't fish them often, instead use them to refine my casting stroke. These lines are a dream to cast.
However I also have the SA steelhead taper which is a similar long taper and fished it hard for years, it's a great fishing line. These lines not only extend the energy in the loop for more reach but they mend very well too.
I fish Spey on rivers now primarily so the singlehander long belly gets little use in fact it's not even on a spool right now for all the Spey lines I use.
Downsides are that they dont load stiff rods unless you have enough line out of the guides because the grains are spread out. The long gentle taper will roll cast very nicely provided you match it with a rod that loads up with the length of line you are rolling. An under-loaded rod will not roll cast very well, you will need to get a flex into the rod to get it to roll with ease.
I would also say that using a heavy fly or casting into a headwind requires a short stout leader and a good compact loop form where a grainy short shooting head would be less work.
I like these lines, they teach the caster to 'feel' the line all the way through the cast which leads to better mechanics.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum