Before we get into this let's get the disclaimers out of the way. Soup curry is a really personal thing. Different people like different places, so it's probably best for everyone to take my recommendations as places worth checking out that I really like. Not that any of that really applies this time since I'm not all that fond of S.
But S is famous, and popular with a lot of people including my girlfriend. Famous enough to have signed autograph cards on their walls from celebrities based outside of Hokkaido (there are quite a few soup curry places around with autographs from local newscasters, sports-people and members of the local TV comedy group TEAM NACS). Famous enough that in an episode of a TV show I saw, when a bunch of really famous comedians came to Hokkaido, S was the place they went to try soup curry. Hell, famous enough that in the new video game Ryuu ga Gotoku 5, apparently you can go to S in Sapporo and have soup curry. They even have a collaborative line of soups! I really want to try that game but sadly don't have the hardware... Or the cash... Oh well...
In fact, instead of posting a real photo of the outside of the place, here's how it looks (outside and inside) in the game. In tiny pictures I found on the interweb:
There's no alleyway there in real life.
Recently the aforementioned TEAM NACS guys made a TV drama series called Soup Curry about a group of friends living in Sapporo, that was tied together with a general soup curry/spice of life theme. They filmed in some real soup curry joints and featured one or two places over the end credits every week. S was one of the ones that showed up there and that was what spurred me into finally trying it out.
S does soup curry and also a thicker, more standard roux curry, along with some tandoori-style chicken. The place is good, and I do like it, but it's not amazing and I'm pretty mystified as to why S is one of the more famous soup curry restaurants in town. The soup is delicious, I mean, it's really, really good, but it's just so thin! I've eaten a lot of soup curry, and for S to be one of those places that folks from outside of Sapporo go to try the dish just seems weird because it's really unlike the majority of soup curry places. Maybe they're just really, really good at self-promotion.
Here's one of their standard soup curries, with their Sapporo-style ramen noodle topping (which is a really nice idea, served with the ramen spoon too, another nice touch) and - c'mon - that just looks like ramen with extra veg.
Here's a half-and-half dish they do of their soup-curry and their roux curry.
For me neither one was satisfying and I didn't really feel like I got my money's worth on the quantity side either. I didn't really like the roux curry, didn't think much of the chicken, and the soup curry was basically a side-serving.
In general I haven't been impressed with the vegetables or the meat there, and the menu is pretty limited. Jesus, looking back at this it reads like I'm basically trashing the place. I'm trying my best not to, and since I started sticking to just basic soup curry with that good noodle topping I've left pretty satisfied a couple of times. But while the soup flavour is wonderful, I find everything else about the place just 'ok'.
S's soup is really light, and that's probably one of the reasons my girlfriend really likes the place. And god knows that it's good to have a range of soup curry places to choose from. You don't always want the kind of thick, heavy flavour-fight you can get at nearby Samurai. So if you want a light, delicious, spicy soup then you should go. But I don't think I'd head back if I didn't eat so often with someone who loves the place. That said, it's certainly no hardship to eat there. That broth is really tasty.
S is really centrally located, about two minutes walk from Susukino station on Eki-mae Dori. It's south of Tanuki Koji, down in the same basement as Hendix Art Cafe, whose sign is pretty visible when you're walking down the street. I'll mark it on my map of course, and would encourage everyone to check it out. You're not kids any more! You've got to make up your own minds about these things.