I've cooled off a bit on scarab. Mainly because it is the style of card that is just terrible against aggro or board control decks. But also because if you already have a 3 drop, scarab isn't a card that is going to pay off well even when on curve. On the other hand, it seems like most of the best 3 drops aren't played on curve anyway. But that also impacts how good of a card it is played on curve... double edged sword.
It is a fun card and shaman seems to have the highest amount of 3 mana cards you'd want to discover, shaman needs stronger board presence on turn 2 than a 1/1. So basically sums up my thoughts earlier in the day before I played with the card a bunch. You probably don't play this over haunted creeper in almost all shaman decks. And I don't think you'll play it over totem golem, especially when shaman is about to get a 1 drop that is buffed by playing totem golem.
There were times when scarab did really well. Getting a third hex, a free totem tuskar, a free feral spirits against face hunter... but I'm not sure if that justifies it. In some sense, you're just hitting hero power on turn 2 and getting a 1/1 totem. Hell, maybe scarab would be worth running if it were in fact a totem race not beast.