Very general answer: bad
More specific answer: you can make good teaser bets if you are selective and discipline.
Basically what Barnstorm said, what i'd add is just remember in a teaser you are buying and paying for every half point. So you need to cross the key #s to try to make it worthwhile. 3 is the big one followed by 7, 10, 6, 4, 14.
The other factor is the total. If the projected total is high then every point is worth less and this is why college isn't a positive bet. I look for games where I can cross the 3,4,6 and 7 with a total under 45.