Using a service that is not a true legal firm may result in an incorrect filing, eventually leading to much higher legal costs to avoid deportation or having to re-file and start the immigration processes over. Many of the immigration attorneys have worked within the immigration department or have extensive experience in helping people entering the United States.
If you do not speak English, look for an immigration attorney that either speaks your language or has translators on staff that can provide language services. In many cases these services are included as the cost of the attorney, but it is always important to ask if translation is billed over and above the attorney fees or if it included.