Vaplaugh provided an incredibly accurate summary of the pros and cons, and was right on the money in saying that the pros outweigh any cons.
I worked at the US Attorneys Office in DC for 6 months as a UG intern. I came in there expecting to be doing document review, pulling discovery info, etc...
What I got was astounding. For the first week or so while I was getting acclimated, they trained me, and had me do basic stuff like discovery. Then one of the attorneys got wind that I was a good writer, and familiar with legal writing, research, and citation. A couple prosecutors came to my desk asking me if I could respond to motions that they couldn't because of a trial, meetings, etc... I was amazed that they would hand that kind of responsibility to a UG intern, but I happily did the first few I got. Apparently they really liked what I did, because from then on every day I came in to find like 10 motions stacked on my desk waiting to be responded to. I also got to basically take charge (with oversight of my mentor of course) of the organization of a substantial fraud case, and was at the bench helping with jury selection 2 weeks later, and sitting at counsel's table through the trial (thanks to the judge who allowed it).
Basically what I'm trying to get across is that I got a lot of substantive and interesting work/responsibility as an undergrad intern. When you have a job there, they actually give you things to do - there's really never any boredom. It's a fantastic place to work, and I hear that many DoJ jobs are very similar in that respect. I would highly recommend looking into it seriously if you are interested.
They don't hire right out of law school typically (at least at USAO), but if you get a clerkship it's very possible to get a job coming off of that. There are a lot of laterals from private firms, but even more from other gov agencies.