I'm actually writing a paper on this topic right now for my pharmaceutical policy class (due tomorrow).
There are mostly cons for online pharmacies, as ernievdb mentioned previously. Another con is that because most online pharmacies operate outside of the United States, the safety of the drug is not guaranteed. Also, the "doctor" that approves your online questionniare and prescribes the medication may not be licensed in your state, if they are even licensed. Not to mention, is is illegal to prescribe without a valid prescription.
I guess ther are some pros for the consumer. Online pharmacies are cheaper, it may be more convienent, it helps them avoid any potential embarrassment that they may feel if they walked into a regular pharmacy, and it increases access to people living in really remote and rural areas.