Assuming you live in America, it is illegal.
Section 1201 of Title 17 of the Digital Millennium Copyright Act (DMCA) defines the circumvention of copy protection in any and all circumstances as copyright infringement and specifically prohibits any circumvention of a technological measure in connection with gaining access to or using a work. This is referred to under the law as a prophylactic regulation, a preventative blanket rule which takes neither context nor intent into account.
MPAA president Jack Valenti defended that interpretation in testimony before the U.S. Congressional Subcommittee on Commerce, Trade and Consumer Protection in 2004. "Back-up copies are not legal," Valenti said. "The Copyright Act does not say 'buy one movie, get one free.' There is no more a right to a back-up copy of a DVD than a back-up DVD player, lawn mower or set of wine glasses."
While you may not agree with Valenti's interpretation, the government does. That is why the Digital Media Consumers Rights Act (DMCRA) has twice been introduced into Congress in an attempt to overturn the DMCA's prohibition against making backups for personal use.
The DMCRA bill was introduced to the United States House of Representatives on January 7, 2003 as H. R. 107 by Rick Boucher. The bill was co-sponsored by John Doolittle, Spencer Bachus and Patrick J. Kennedy.
The bill was reintroduced into Congress once again on March 9, 2005 as H. R. 1201, the 'Digital Media Consumers Rights Act of 2005'. The 2005 bill's original co-sponsors were John Doolittle and Joe Barton.
The authors of the bill summarized the DMCRA, in part, as intended to supersede the "Fair Use" definition under the DCMA by stating that "it is not a violation of Section 1201 of Title 17 to circumvent a technological measure in connection with gaining access to or using a work if the circumvention does not result in an infringement of the copyright in the work."
Due largely to intense opposition from copyright holders such as the Motion Picture Association of America (MPAA), the Recording Industry Association of America (RIAA) and the Business Software Alliance (BSA), the DMCRA bill was never even brought before Congress for a vote. Thus, the prevailing legal opinion is that Section 1201 of Title 17 of the DMCA remains the controlling statutory authority in this regard.
Like it or not, that's the law. Google "case law DMCA" or "321 Studios DMCA" if you want to see for yourself the legal opinions, case briefs, rulings, etc. which define your rights (or lack thereof).