Google penalties are not fun. We have personally spoke with people who have received a slap on the wrist from Google and the cleanup process can be very long and painful. Depending on how bad your situation is you could find yourself really trying to hunt down the culprit responsible for pushing you into the hot seat with Google and it can take some time. If you depend on your website traffic for revenue and company growth you will know that a penalty can really keep you up at night and utterly cripple you.
At some point during your journey you will definitely have to do something called a reconsideration request with Google. This is something that can be done directly through Google Webmaster Tools once you feel you have cleaned up your dirty steps you might have taken marketing your business.
Here is a great video from Matt Cutts discussing the topic in further detail.