MSN ppc traffic sucks, they have too many bugs in their system. Their control panel is not user friendly, and they overcharged my account by $500. I had only intended to use the $100 voucher but somehow their system did not tell my campaign to stop. It took a month, but I got it figured out. Adwords is too damn expensive. Miva sucks. Spent $250 with no conversions and highly targeted long tail keywords, good adcopy, and inexpensive clicks. Adbrite ppc...hahahahahaha...absolutely the worst! Unfortunately your best bet is adwords. Its expensive but you will convert more using google and paying for 100 clicks versus 10,000 clicks on miva and adbrite.
Dont just go for cheap!