There are literally hundreds of company search optimization firms in the world, all doing pretty much the same things. There are only a few different techniques with regards to the process that have any effect at all on the actual rankings of a website, and all of the rest simply does nothing, or possibly might even hurt the rankings if the company is not careful. When you consider who to hire, you must look at the results that they have achieved for clients over time, and then consider if the results have shifted as a result of changes that Google has made in the way it indexes. The best companies are not going to have their clients effected by changes due to the fact that they are actually producing real content and links, all within the terms of service of the major search engines. It is when you attempt to do things that are not allowed that you lose rankings.