So, due to SS1's being the default rental/class gear at the shop I used to teach for, I switched to using one when I started teaching (when we first got certified there I had to have the 'discussion' with them on buying a traditional octo setup when we acquired our gear). I'm now teaching for a shop that's more tech oriented (octo, SPG, even BP/W in the rental fleet) so I've been evaluating my setup again.
Teaching means that I regularly get to do (simulated) out-of-air ascents. In general, I have not found it any more difficult to do an OOA ascent while breating off the SS1 since in *that* situation, we are vertical (linked up with the OOA diver) and already doing to look-up and keep the inflator hand "up" things. Because of being vertical air vents well, though I do notice it venting into the mouthpiece (and eventually out of the mushroom valve). I'm venting just using the deflate button, not pulling on the hose/cord.
If I'm horizontal, it would probably be a pain to vent. In that case I might consider going head-down and using a lower dump valve instead. That would likely only happen if for some reason I'm breathing my SS1 while not donating my primary though, which is not impossible but not (IMO) very likely, but could happen with something like a mushroom valve or mouthpiece failure. Going slightly heads up it vents fine though.
Things I like about the SS1 are the nice large buttons - they're easy to find and operate even with gloves. One less hose cleans up the rig a little, which is a couple less failure points. It's not specific to the SS1 by any means, but using the garden-hose-thread adaptor makes removing it for cleaning, switching between different BC's, and cleaning the BC easy. I can clean my wing bladder by hooking up a hose and letting it run until the OPV's open. I have some Zeagle non-breathable power inflators with the same hookup that I can swap in quickly if I want do switch to a traditional octo setup. It also doesn't require any kind of "Octo holder" (there's some out there that show up in class that look like they're more trouble than they're worth to operate... the cheap/simple one that look like a slightly oversized snorkel keeper seem to be the best)
Dislikes? Seems a little harder to adjust - not sure if there's an inline adjuster for it like normal regs can take to do fine adjustments. It's a different service kit than the other 2nd stages. And yeah, it is indeed a pretty short leash when breathing off of it - I would not want to have to do a long transit or stops that way. A couple/few of minutes doing a nice slow recreational dive ascent is not a big deal though. It means I need to be more self-sufficient on spares, since it's more involved to switch to a loaner/rental reg set
I also use it as a demo to my students to show what the might see out in the wild that differs from the gear they're wearing in class, and to explain one of the reasons why we teach primary donation.