1) Depends on what you want to do when you graduate. Generally, pretty important.
2) Federal is considered more prestigious. If you have a specific state in which you wish to work, a state supreme or appellate court clerkship in that state might trump a federal clerkship on the other side of the country. It would definitely trump a magistrate clerkship, possibly a district court clerkship, but definitely not a COA clerkship.
3) It looks OK. It's better than some things, not as good as others. All depends on the office you work in. But, forget how it looks. I really enjoyed my time there immensely, and highly recommend it to anyone.
4) Familiarity with the federal government is always somewhat helpful in applying for jobs with federal agencies. If you worked at the USAO, did well in Admin Law, etc., that will be important. It will not trump good grades though.
5) What about it? If you like it and they pay you and the work is fun, then do it.
Generally agree with all of this but I wouldn't say clerking is "important" as much as it is really, really helpful. Clerkships (even at the state level) are hard to come by and no one iwll hold it against you for not having one. But, if you want to be a litigator, clerkships can be a tremendous advatnage.