I've used both the SA 5-speed and 8-speed. The 5-speed takes a little while to break in and can be a little rough in first and fifth gear while it does that, but I've never had any trouble with it other than that. The 8-speed is kind of weird, I think it was intended for use on small-wheeled bikes with the gearing arranged the way it is, but with a 32-tooth chainring and a 25-tooth sprocket it works OK with the 700c wheels I'm using it with, and I reckon I could use an even smaller chainring if I wanted.
One thing that probably is worth mentioning is that the 5-speed ticks in the top 3 gears as part of the way the mechanism works, and the 8-speed ticks in all of them but the lowest. With the 5-speed it's a relatively slow tick, which I'm used to from old SA 3-speeds, so I barely even notice it, but the 8-speed ticks much faster, and although it's quieter, the ticking is much faster, and because of that, when the hub's in certain gears it almost gives that rough feeling like the chain needs oiling. That's only the internal pawls that give that sensation though, I don't think it's the actual gearing running roughly. I've only been using my X-RD8 for about a month, and I'm basically used to the feeling now.