SEED Destiny was rather meh. It had some nice moments, even a few kinda cool characters such as Gilbert Durandal and Meryin Hawke. The "main character" of the series, Shinn Asuka is the most unpleasant thing that Gundam had had to deal with in 10 years. I feel that like Shakti Kareen he is actually the main antagonist of the series in and that the greatest number of protagonists die as a direct result of their actions even if they didn't kill them personally.
Personality-wise Shinn is horrid. I hear that many people compare him to Kamille Bidan. This comparison isn't entirely inaccurate as their behaviors and attitudes towards the begining of their respective series were very similar, save one key detail. Even at his most whinney Kamille's arguements about the corruption and depravity of adult society were extremely valid, whereas Shinn is a complete hypocrite who's weak arguements collapse on themselves. He also pilots a robot made up of combining pieces which reminds me of the -shudder- Victory Gundam. I never liked combiners (or really giant robots for that matter) but I accept them, they sometimes have a bit of a function. The Destiny and Victory Gundams do not (at least Victory had the excuse of being mass produced, henced more of a function to modular parts and also apparently the best way to own something is to launch off your legs at it) have a function in their combining. It serves no purpose except to waste time and look stupid.
Whew, enough about that, really it wasn't the mech that bothered me. The main characters of Gundam SEED Destiny are clearly Athuran, with Kira and Cagali as the next most important supporting characters. Shinn's whole function is to mess with Athuran's head enough that he doesn't leave ZAFT earlier in the series. While Kamille matures throughout Zeta and ZZ Gundam (in Z because of a near death experience and lots of slapping and in ZZ as a remarkable recovery of his broken mind), Shinn simply sinks deeper into the pits of circular logic, immaturity and emotional overreaction.
In the end I think Shinn and Shakti are the two most unpleasant characters in the history of Gundam ever.
Other than the overwhelming annoyance of the "protagonist" SEED Destiny was an alright series, however it is definately one of the weakest Gundam series. Nothing significant changes with the main characters, save perhaps Cagali and her developement is a bit disappointing. Kira sits around doing nothing until he is forced not to, then he goes out and kicks ass. Athuran for some incomprehensible reason reverts to the "ZAFT is always right" attitude he had in SEED and thus fights Kira, like he did in SEED. Key to this regression and lack of progression of character is Shinn's amazing aura of negative influence (look at what he did to Lunamaria and Stella). Cagali seems to wimp out really bad early series and has to be forced into regaining the strength she showed in SEED. Murre Ramius continues to be the best and most awsome captain ever (yes I know she's not Bright, but despite Bright's awsome her skills are still superior), Mu La Flaga gets magic memory wipe and assumes his logical role as the masked character, and Lacus Clyne continues to sit around doing basically nothing until the end of the series then she goes "I WILL TALK AND YOU WILL LISTEN!!!!" and the universe obeys.
My conclusion. The general comparisons are of SEED to the original Gundam and SEED Destiny to Zeta Gundam. I disagree. SEED is more like Zeta, SEED Destiny is more like Victory, except without most of what made it bad, but equally pointless. I find it worth watching just to have more Cosmic Era and a few Gundam self-referencing giggles, but that's really about it. Not something to watch for it's own real value.