The pros:
Convenient
Fast
Easy
Cheap
Versatile-you can change your the weft of hair anytime without buying a whole new clip.
Good for lazy, rushed days, hiding your hair, baggying those ends, etc.
The cons:
Honestly, I don't see any cons. I know others have complained about the bulkiness, but it does not bother me any.
I've been wearing mine every day for almost a year now and I love it. This is great since I do not plan on wearing my hair out until I reach my next goal, which may be achieved in April or so of 2009. So The Mane Cli is my best friend, I love it!