i edge with just a brush all the time, but if you want to do a good job, you need a good brush, and a steady hand. If you use the cheap brushes, you might save money, but you'll lose time and the results won't be nearly as good. Believe me, I m cheap, and try to save money wherever i can, but sometime saving money in the long run requires that you spend some up front.
A couple years ago, someone here shared a tip that involved laying a bead of caulk in the inside joint between the wall and ceiling, and running your brush along that. Can't remember the particulars, but I tried it, and it worked pretty well, but haven't even thought about it again until just now. do a search, it might have be a member named Leah Francis who gave the tip or asked the question, for some reason i think she was involved....
EDIT: i just read a couple threads down, and the caulk tip was right there, in another thread from this week.
Here ya go : ceiling paint