I am also from Europe and did a 6-month internship in California. But for the J1 visa, insurances etc. I happily paid for the services of CIEE e.V. (Council for International Educational Exchange). I'm not sure if this agency still exists, the internship was in 2000/2001, but there are similar agencies around. I had to fill out some forms and have the professor at my university and the manager of the company write some statements about what my job will be and why I am adequate for an internship in the US. But overall the application went pretty smooth for me.
The company I worked for is a global player and has a branch in my home country. But it wasn't possible to apply for an internship in the US through the local subsidiary, so I applied directly at the US branch.
A big advantage has been that a fellow student of my university was doing an internship at that same company the semester before I started my internship and recommended me to his manager. I applied for an internship at other companies via E-Mail or Web, but never heard from them.
The preparation for the intership took about three month with all the sending and receiving of forms and getting all the papers needed, like a clearance certificate (this was pre 9/11!), English language certificate, etc. pp.
Without the help of an agency I'm pretty sure it would have taken me a year of preparation, too. I also agree that the internship should last 6 months minimum to be of benefit for both sides, the company and the student.
But it has been totally worth the money and effort, one of the greatest experiences I made in my life - professionally and otherwise.