Typically, you need to worry about companies that promise a top position in Google's organic or free listings. But Google also warns that you should be wary of those companies that will guarantee the top position in their paid listings, Google AdWords.
Googler, AdWordsPro, said in a Google Groups thread:
Bottom line, if anyone says they can 'guarantee' top placement - whether in the unpaid search results or within AdWords - then buyer beware.
Why? Well, Google's AdWords algorithm is not simply based on how much you pay. Other factors include your click through rate, the quality of your pages and many more criteria. This also now applies to Yahoo Search Marketing and Microsoft adCenter sponsored listings.
Forum discussion at Google Groups.