You need at least one of the following criteria:

1. Deep stacks (so when you hit big that one time, you get paid sufficiently)
2. Multipot (same reason as above, the one time you win you take down a sizeable pot)
3. A guy with "Aces" or other obvious big ones who raises big pre-flop (you will get paid big the one, rare time you hit, since overpairs are usually overplayed)

I guess the reasoning is the same behind all of the conditions. When you play crap, you're odds of winning at the showdown are low. However, the rare instances that your crap hits a winner, you will be paid big (in theory, due to multipot inflation or deception) Suited connectors are the best "crap" (unpaired baby cards) b/c they have much much higher odds of making straights and flushes by the river. (gappers and 2-gappers are significantly worse, so conditions must be that much more favorable)

It's a pretty subjective decision to start playing SCs. I guess you should use some software to see if you're profiting if you can't get a feel for it.

Generally, if you're in some MTT or SNG and the blinds are really big, everyone's tense, and hands never make it past the flop, you will not want to touch SC's and other such hands (22 for example, which works by the same principles)

When you're playing a deep cash game, it's much better. You can afford to screw around with drawing hands and other garbage, knowing that the payoffs can be that much larger.

SC's also relieve boredom