Many firms that i've personaly worked together with didn't really deliver what they claimed... But it also seem's like that most search engine optimization companies don't have a clue about what they are doing (or even worser, they are outsourcing their work over-seas).
What i've figured out is that a proper interview can tell you a lot about the company that you plan to hire for your website improvement & rankings. Since SEO has changed over the last years, many firms have been crushed by the so called "Panda & Penguin" Updates... They had no answer to those updates, so everything that they could do is to let their clients fall, right after those clients got their websites penalized..
...This is propaply not what you expect from a SEO company, isn't it?
When it comes down to search engine optimization, one important key to success is to be up-to-date. Most companys hire college graduated students that have no clue about how ranking websites really work (because they didn't even ranked a single one, not to speak their own sites).
Keep in mind to make sure that a company can actually hold what they claim... SEO still works, but only if it's done the right way. One of my friends in germany had really great success with hiring seonuernberg.org - Webseite. They've delivered extraordinary results and a fast turnaround time. To learn more about their goals for your business aswell as their ambitions, take a look at http://seonuernberg.org/ziel/
You can see that there are still companies out there (and also in germany) that can produce fast and professional results. If you are still worried about outsourcing SEO to a company, you can also learn it by yourself, which will propaply take more time and more money then hiring a professional agency.