Well, this is a personal thing but I quite like it when companies use more detailed art for the packaging rather than a direct representation of the animation style in the show. There are some examples which are sort of weird (the old Viz DVD covers for Ceres Celestial Legend used Watase Yuu's floaty shoujo art which looks incredible but nothing like the cheap anime style of the series itself). But then you have things like the Diabolik Lovers DVD cases in Japan, where the LE artwork uses the original illustrator's work and they look so much better than the anime designs that I can't open my wallet fast enough.
Nothing like the show's art style:
http://www.amazon.co.jp/DIABOLIK-LOVERS ... 00KFSW58M/
The accurate version:
http://www.amazon.co.jp/%E3%82%A2%E3%83 ... 00EF6TK74/
The Durarara!! x2 covers are supposedly by the anime's actual character designer, so while it looks as though he was going for a more stylised look to match Yasuda's illustrations I think if anything the television artwork is probably the thing falling short due to budget constraints. For what it's worth I really like the covers.