In terms of the series, episode 26 was a decent ending. It didn't fix the mess of the last ten episodes, but it brought resolution to the main points of the plot.
Hayate and Frejya make sense and them together is an earned conclusion that I'm happy with. I'm really disappointed that Mirage got sidelined as much as she did through the series, but I'm also pretty happy with her seeing Hayate as more of a brother/friend/partner than a lover.
Delta took the Lost approach on its main mysteries, and that's frustrating, but I'm very relieved that we didn't have a last minute Lady M ass-pull cameo as episode 25 seemed to hint towards. At this point in the series, that would've been a much bigger disappointment than leaving it unresolved.
The action was fairly low key, but I actually don't mind that. A big fleet battle with wall to wall dogfights would've felt like too much of a retread of Frontier. Since they borrowed the hive mind aspect already, I'm glad they kept the focus on the character growth for what won the day. The Elysion looked good doing its thing and the action that we got was fun.
Bogue being pissed off that he had to help Walkure was funny and fit nicely. It actually made me respect him a bit more (not a lot, but some) that he followed orders despite his personal feelings. I would have liked to get a Hermann/Mirage interaction to complete that circle (which started all the way back in the asteroid battle of episode 6), but you can't have everything. The Keith/Roid conclusion mostly fell flat for me, but then they've both been such poorly realized characters that this was hardly a surprise.
Does Delta go down as a success? Debatable. The missed opportunities with the characters and story are pretty glaring and frustrating, and the storytelling choices and pacing in the second half leave me scratching my head. That said, there were an awful lot of things it did really well (at least, most of the time). Frontier was a better story, but Delta has better characters and is an interesting departure from the Macross formula. Maybe too much of a departure, but I won't fault it for trying. If there's a movie, a DYRL style retelling that trims out the exposition and brings Mirage out of the shadows would make this a damn good tale.