There are 3 time periods and the first two are fine, the third is shorter than it leads you to believe, you soon find yourself out of time when you thought you had plenty which can make a massive difference to your ending, as I discovered to my chagrin.

I actually like it... Though, granted, I had the added-explanation ending DLC installed. What I like the most is how it gives you a seriously uncomfortable and ambiguous choice at the end, to the point where even the "screw you guys, I'm not using this after all" ending has compelling arguments for it.

For me, at the end of a pretty much perfect mostly-Paragon run of PS3 ME2 and ME3, the only viable choice for me was in fact NOT the DNA-merging ending, but the Control ending (martyr yourself into a more beneficial AI that then controls the Reapers as a helpful, all-powerful space police). Though I did have all of them available.
While in that ending you still had to trust that the AI made from you wouldn't go crazy and ruin everything anyway, ANY ending using the Crucible has you trusting that it'll do what it's supposed to, as explained by an AI that's using flawed logic anyway. (Which is what makes the "screw you, I'm not using this after all" ending viable.)
So out of the three evils of "destroy the Crucible and also every single AI in the universe, including those you helped bring up as a proper lifeform", "force genetic change that could go awfully wrong (and also possibly a hive-mind) on everyone" and "potentially totalitarian Shep-AI space police", I chose the one that left everyone but the Reapers technically untouched. That way, even though they were being guided and helped along by the Shep-AI, they were still themselves, running on their own devices, with everyone still alive to take up the fight again if things should go wrong.
The "Control" ending was, to me, the safest course, and the one with the least sacrifice and risk required on the part of the entire rest of the known universe. And since it'd been made pretty damn clear throughout the entire game that the only way the current spacefaring races would survive would be to use the Crucible, it HAD to be used.
I know the "best" ending is supposed to be the "synthesis" ending, which is only available if you get past a certain galactic readiness rating. And I had it available. But, seriously... Changing the genetics of every single species out there? Linking them up with the Reapers in what both the Crucible AI and EDI in the epilogue hint at as being a hive-mind? The other three options would have to be SERIOUSLY tasteless to me for me to go for that.

The 'screw you I'm not using that' ending was something they added in the revised ending patch, you didn't have access to that originally.

I went for the synthesis ending, cause to me it felt like it fit the fiction best. The other choices felt like I was just 'restarting' the cycle again, but that felt like a genuine way out of the cycle of destruction and rebirth the Reapers represented. That, and I wanted Joker and EDI to be able to have kids... hehe

To me, the Reaper/Crucible AI's reasoning for the "cycle" was both flawed and self-fulfilling. Throughout ME2 and ME3 (I dunno how much that pops up in ME1, only having played the comic synopsis at the start of PS3 ME2) the idea is established that synthetic life can be equal to non-synthetic life, at least as long as you've been playing Paragon.

The Crucible AI basically started wars between synthetics (the Reapers themselves and controlled AI critters like the Geth) and whichever races were advanced enough, BECAUSE it thought that AI destroying all organics was inevitable. Paragon Shep disproves that several times over if you play carefully. And in fact, when peace is made between the Quarians and Geth, it can be argued that they're already headed towards Synthesis.

The "Control" ending does leave the door open for, say, the Shep AI to go nuts and restart the cycle. But if you subscribe to the idea that a significantly advanced AI is no different from a living person, then I see only one reason why the Synthesis ending would avoid future wars (besides the whole "well they're all organic-synthetics anyway so it wouldn't be a war between organics and synthetics but simply a regular war so the cycle didn't actually restart" argument, which is just semantics). That's if the Synthesis ending also actually introduces the hive-mind it hints at.

And hey, instant telepathy across the galaxy sounds neat. I just wanna make sure people can turn it off at will before I push the button (or throw myself into the beam) that enables it. And not forcing everyone and everything to have circuits in'em in the process would also be neat.