Until this season "Sunny" has gotten progressively better each year -- finally hitting its apex with the amazing Season 5. Season 6, however, seemed to deviate from this momentum and proved to be generally uneven.
As some other reviewers have noted, only two episodes were actually written by members of the main cast ("The Gang Buys a Boat" and "Who Got Dee Pregnant?") which fittingly are two of the season's strongest episodes. This fact points to probably the most glaring problem with Season 6 -- it has great moments, just fewer solid complete episodes than seasons past.
Sure "the implication," "Lethal Weapon 5" and Mac calling everyone a "Jabroni" will easily enter the canon of great and memorable Sunny moments, however episodes like "Mac and Charlie: White Trash" hardly hold up against their catalog of better written episodes.
Maybe it was too many side projects, maybe it was working around Kaitlin's pregnacy, maybe it was not having Fred Savage behind the camera or Rob, Charlie and Glenn not actually writing most of the material that hindered Season 6, but one can only hope Season 7 will regain the remarkable momentum this show had formerly achieved.
As for the consumer, I personally will buy this season just because I've watched the first five so many times -- but ulitimately I'm sure I'll be pulling out Season 6 discs as often as Season 1 in the near future. Funny moments, but few great episodes in this DVD pack.