In the end his own dissertation advisor called him more or less a fraud and you could see on his face, when he sat in the parliament, that the whole thing was nagging at him more, than he would admit and show in front of the camera or at the speaker's desk.
I think he is a bit arrogant, maybe because of his aristocratic heritage, but I also always felt he was a moral person at the core and also a very talented politician. I bet he would have resigned much earlier, if it wasn't such a critical phase for his party CDU. There are lots of elections this year, very critical ones among them, and Guttenberg is/was the CDU's biggest hope for the future. I'm sure that many of his CDU friends told him to hold out, no matter the cost, but in the end that wasn't a realistical option anymore.
So him resigning now is a disaster for the CDU but it probably isn't his career's end. I would have hoped, that his resignation was more dignified and not only a result of the enormous pressure put onto him, but at least his career might recover from it now and some day he can come back stronger. If he hadn't resigned, though, he would have never heard the end of it. It would have been an eternal open wound for journalists and academics to lay their fingers on.
I believe, that in the end even chancellor Merkel (she's an academic, too) must have realized, that there is no way out of this situation. Guttenberg's popularity is still strong but with the ongoing barrage of fire from the media, it would have only been a question of time, until it sank. This way he can remain popular and maybe rise again in the future -- maybe even as the next CDU's candidate for chancellor.
But again it shows, that the conservative parties' leading politicians are much more clinging to their seats, when they have done something wrong, than their counterparts from the social-democratic SPD for example.