I have a love/hate relationship with Days, mostly because I have a love/hate relationship with Xion (I do like her, but there are aspects of that makes me want to whack her over the head with her Keyblade; I won't get into that though, as this thread isn't about Xion). I like the game, but I also find it to be completely unnecessary. See, KH2 already explained why Roxas left the Organization (Roxas never knew about his origins, got PO'd by it and left because he wanted to meet Sora; Riku said as much to Sora: "I think he left the Organization because he really wanted to meet you.") and duel-wielding isn't really a big enough issue to need an entire game to explain it, nor did it need Xion to exist for duel-wielding to be possible.
And yeah, the missions were rather repetitive. Some were fun, others were just ugh (I also want to know why the Org decided to explore worlds that they never actually used for their purpose; why did they go to Wonderland? To collect Heartless? Then why not also go to other worlds with Heartless as well, like Traverse Town? Why didn't we see them set up for some events in KH2, like what Xigbar did in the Land of Dragons, or what Luxord did in Port Royal? The list goes on...). Loved the panel system, even if it was frustrating sometimes. xD
Also, while I felt the derailing of Roxas' friendships sad, I found his codependency that magically disappeared in KH2 rather annoying. At the end of Days, he was still heavily clinging to the friendships he had. In KH2, while his friends (HPO) were the source of his happiness in Twilight Town, he wasn't dependent on them. He clung to them, but they didn't define his every action like his friendships in Days did (example: At the end of Days, Roxas was fighting for his existence, it was all for the sake of seeing Xion again; at the end of his story in KH2, Roxas was fighting for his existence, not for his friends' sakes, but for his own).
The scrip was pretty good too, and graphics weren't half bad for a DS game.
Overall, it's a good game, but it's one of the weaker game plots of the series (not the weakest, though; that title goes to RE:Coded).