First, the climbing in Spearfish Canyon is absolutely stellar. That being said, if you want to know what climb you're throwing yourself at, or even find some of the climbs to begin with, I highly recommend buying the guidebook, in addition to downloading mountainproject on your phone.
Guidebook = better approach directions, color pictures so you know what wall you're at, warm fuzzies for supporting a local climber who's put a lot of work into developing the area
Mountainproject = lists routes that aren't in the book , has some of the best route descriptions I have ever read. Seriously, if you're having a bad day go read route descriptions like this and laugh until whatever you're drinking squirts out your nose.
Now that you know how to find the climbs, I hope you have a brass pair of cojones to help put them up. Or a stick clip. There are lots of places I haven't climbed , so maybe these observations are only relevant to my own, limited, experiences, but this is the first climbing excursion I've been on where I was really grateful to have a stick clip around. For many climbs, if you come off the wall before you clip the first bolt you risk not just falling to the ground, but potentially falling off the steep sides of the trails. Not a pleasant prospect.
I also found the climbing at Spearfish to be rather challenging for the grade. When the rating says 5.10d, it doesn't mean there's one crux move that feels 5.10d surrounded by mostly <5.10 moves. If it says 5.10d, be prepared for every single move to feel at least 5.10d, possibly including the ones to get off the ground. I can't imagine how strong the local climbing crew must be if they're putting up climbs like that all the time. I usually felt like I had limp dicks for arms by the end of every climb.
A note on the approaches: some are short, some are long, all are steep. If you're the kind of person who thinks all sport climbing approaches can be done in flip flops I highly recommend you dig out some real shoes with laces for this trip. Forget switchbacks, it looks like whoever put in these trails were so excited to climb that they just wanted to get to the crag as fast as possible, which means going in a straight, steep line.
All said and done, I want to reiterate that the climbing at Spearfish was awesome, and I wish the canyon was just a wee bit closer to home so I could visit more often. Technical pocket sequences, pumpy jugs, and itty bitty crimps, Spearfish has it all and I can't wait to go back.