I did externships for credit with a federal district court judge and a Sixth Circuit judge. I don't think it affects the nature of the work you'll do if you're an intern or an extern - but why not take the credit and not have to take another class in school?
To those of you lucky to land these fantastic opportunities, don't squander it. Work as many days as you can. Be hungry for the most challenging work you can get. Also, make sure to take the time to observe hearings, even if you haven't worked on the case involved.
I was surprised at other externs who just worked the minimum number of days and hours, acting like this was just a line for the resume and not a fantastic opportunity to learn from a sitting federal judge. I stayed the entire summer - weeks longer than anyone else - and worked as often as I could. How often are you going to get to work closely with a federal judge (barring, of course, clerking post law school)?
The district court judge I worked for (and his clerks) understood that this was a learning experience and were happy to allow me to spend hours observing various hearings, or trials that were occurring in the building. After you've worked on basic assignments and proven you're not an idiot, be proactive and ask the clerks for the opportunity to work on more substantive matters. I had the great pleasure, near the end of the district court internship, to work closely with the Judge, drafting memoranda on a declaratory judgment matter directly for him. He even asked for my recommendations, which happened to conform with his view of the case. The Judge's personal "nice work" was an amazing feeling. He ruled on the case on the last day of my internship, and explained to me how even the loser in the case was happy, as they now knew there was no real threat of future litigation.
The Sixth Circuit externship offered the opportunity to draft substantive bench memoranda just like the appellate clerks were drafting, but it was much more of a sit at your computer working gig, rather than the hustle and bustle of the trial court.
The district court externship taught me a lot, not only from the substantive work I did, but seeing the reality of what happens behind the scenes in the Judge's chambers. There's the "law," and then there are the realities of the administration of justice. I was always amazed at the poor quality of some lawyers' pleadings. It's a confidence booster. Judges remember lawyers, know who the assholes are, and lets just say it's true - your reputation is important, and the legal community is small.
So good luck to you seeking these externships, and to those of you privileged enough to land the opportunity - MILK IT FOR ALL IT IS WORTH.