Much like the benefits of offshore drilling, the disadvantages are also hard to predict when it comes to probability and impact. A lot more will probably be known months from now when a report can be done on the economic and environmental impact of the current spill.
What is known is that the benefits of offshore drilling are limited and short term even under the best case scenario. There is a relatively small amount of oil off the coasts of the United States and the economic benefits are likely to be spread out to countries outside the United States. On the other hand, the disadvantages are very large and can last over a longer period of time. The "tiebreaker" in this equation would likely be the environmental impact of offshore drilling. If one gives little to no value to the environment then offshore drilling probably provides a net benefit as a purely economic activity. On the opposite end, if one values the environment more than the limited economic benefits of offshore drilling are probably not worth the risks involved.