A typical usage is to praise people for living up to the expectations of others. As an example, early in Chrono Trigger, a prototype teleporter malfunction results in a young woman vanishing, so Crono, the protagonist, picks up the pendant that apparently triggered the malfunction and steps onto the device, prompting this line: 「後を追うってのかクロノ。さすがは男だぜ！」 ("So, you're going after her, Crono. Way to be a man!"). Putting aside the question of whether this actually has anything to do with being a man, the speaker, at least, has certain beliefs about what a real man is supposed to do in a situation like this, and Crono's actions meet with his approval.
さすが can also be used, often in a form like さすがの～も or さすがに～も and in a negative sentence, where it indicates that not even someone like that is capable of this particular feat, highlighting how difficult or otherwise extreme the action would be. "You may be good," it says, "but you're not that good." Or bad, since it can also express depths of evil even the villains wouldn't stoop to, like 「さすがの暗殺者でもあんなことはしない」 ("Even an assassin [who might be thought capable of anything] wouldn't do something like that"), or just simply weird, like if you want to call something so bizarre that not even your friend with the peculiar tastes would like it. To use an example that isn't in a negative sentence, take this line from an Ar tonelico 2 Cosmosphere: 「だって、この世界に来て早々「生け贄になれ！」なんて言ったら、さすがの貴方でも逃げるでしょ？」 ("I mean, if I said, 'Be a sacrifice!' right after you came to this world, even you [regardless of however brave and loyal you may be] would run away, right?").
In a related nuance, さすがに can be used in a sense that roughly means things aren't likely to work out as well as hoped, or similarly, something like "I had thought I would be capable of more than I'm turning out to be". So 「さすがに入りたくない」 corresponds to something like "[Maybe I'm wimping out, but] I really don't want to go in there." It also overlaps with やっぱり to some extent, though these cases feel more like "it's hardly surprising that X" than "X, just as expected" to me... though I'm not sure I can explain why.
To give a brief comparison of やっぱり and さすが,
- 「やっぱり行きたくない」 feels more like: I wasn't really interested in going to begin with, but I thought it over a bit anyway. After some consideration, I still don't want to go.
- 「さすがに行きたくない」 feels more like: Most people in my position wouldn't want to go, and maybe I'm being disappointingly average—or simply sane!—but neither do I.
- 「やっぱり勇者ですね」: Well, that settles it. Some may have had their doubts, but it turns out he really is a hero.
- 「さすがは勇者ですね」: He's a hero, all right, and he's just gone and done something excitingly heroic again.