If you have a more "generic" colored bike, the Testors paint markers work pretty well, too. I've used the black, a no brainer, and the red on frames where it wouldn't clash a great deal.

Layering works well, and you'll have to deal with the buildup, but that's not hard. For filling in scratches, I've sprayed into a cup and brushed the paint on with a Q-tip. The paint will fill in the scratch nicely, but it's thin and needs several coats.

Once the paint builds up a bit, and dries well (I wait at least 3-4 days), you can use rubbing compound to smooth out the touchup spots. I don't spot clearcoat over areas I've rubbed down, because then I've got a 2nd buildup area that I can't rub down much. Some places you can clear with hobby spray clear and they'll come out fine. Other places are better left alone. There's no hard/fast rule on that.

I prefer a carefully touched up frame to a patina'd scratch and ding one any day.
If a frame is so damaged that tons of touchup is needed, or has been done and it's ugly obvious, I pass.

There's an amount of damage where too much touchup makes it worse. Unless the bike is black, a lot of non-matching touchup looks just as bad. Like many folks, though, I've gotten in a frame where something has to be done, and a repaint is not an efficient option. The only answer is touchup, and it can get very tedious. It's worth it, though.