A lot of folks are focusing on your circumstances at your former job (work not being valued, etc), but I don't find that terribly relevant given the way you've handled leaving your former position.
> How would you feel if a fellow developer left in the circumstances described?
About you, personally? Probably a little jealous, but I wouldn't feel that you have done something wrong. It sounds like you gave adequate notice and planned on leaving one week prior to it going production. You then extended that and even that didn't work out. Assuming you're being paid more at this future employer, you've accepted a financial loss for the sake of your team. That's way more than most people do in your circumstances. Deadlines slip, sometimes, but some deadlines cannot be avoided. You made a commitment to another employer, asked for and received an extension to your start date, and now you're at the point where you need to live up to that commitment.
> If you were a dev manager or hired this developer, how would you feel about them leaving in the circumstances described?
You weren't clear on which dev manager (the new one who just hired you or the one which you're leaving) so here's my thoughts: As your former dev manager, I'd be perfectly fine with this for the reasons I indicated, above. There's only so much that I can expect from you and you've already gone further than most. As for your future dev manager, I'm pleased that you cared enough about your former job (and I probably know that you already weren't happy with this project and you still went over and above to try to leave them in the right position). I'm happy knowing that you showed dedication to a "bad project" (and we have bad projects here, too) and am looking forward to having someone on the team with that level of commitment.
Frankly, you already did everything I would have done, here. I would have told my future employer that I accepted the position with a firm start date, and that I will live up to that commitment, however, an important project where I am a key member slipped and would ask for another week to ensure I'm leaving my former employer in a good place (implying dedication and commitment that can be expected from me in my future position). If accepted (and it almost certainly would be), I would work to that date and leave regardless of the project state (working as many hours as required to push through and make it happen -- but we all know that isn't a guarantee). Accept that you've done the right thing and keep your commitment with your future employer. Also, keep in mind, that your former employer almost certainly would not give you this level of commitment if your position were eliminated.