All you basically need to know is: this place's ice cream is awesome. Go there now.
I can provide other information, I suppose. In my explorations, I walked by this unassuming storefront this winter. The place looked totally abandoned; there were only chairs and empty freezers inside. I wasn't hopeful for its opening, but one March day, I walked by and the place was hopping! I wandered in, and I almost wish I hadn't, because now I know how delicious the Eis, or ice cream, is here.
I'm quite sure they make it themselves. There is an extensive array of flavors; I'd have to say that there are at least about 30 flavors or so. They range from fruit ices like melon, strawberry, etc. to super sweet and chocolatey treats like Snickers, mocha, etc. My absolute favorite is sahne Kirsch, which is cream cherry. I also love the raspberry yoghurt flavor, too. I'll be honest: they do these two flavors so well that I haven't been able to stray from ordering them! Friends have ordered a kiwi ice cream sundae, covered with kiwi fruit and kiwi fruit sauce (holy kiwi fruit, Batman!) as well as some chocolate based ice creams and have raved about them, too. Everything has such a vibrant and true flavor, if that makes sense. The ingredients taste fresh and very real.
Prices for take away ice cream cones are very reasonable; it's 50 cents per Kugel (scoop). I think that part of the reason is that this shop is not in the main downtown pedestrian area; it's between there and the Kammgarn.
There are a few bistro tables and chairs inside to sit and some outside as well, but usually people just take their cones with them.
I think that the shop is family owned; the same lady is always there. She's not super friendly to me; I know that my German is awful at this point, but hey, at least I try. However, I don't care. I'm there for the cold ice cream, not the warm fuzzies. Don't let this deter you; the ice cream is absolutely delicious, fresh, and reasonably priced at Eiscafé Rialto.