When I first began using smartphones, my first one, the HTC Hermes, had a D-Pad. I have to admit, I was addicted to it instantly.
It made everything so simple. Controlling music, scrolling, and notably J2ME apps/games all worked better with it. When I had upgraded to a Fuze, I lost the D-Pad, and did miss it really badly at first. However, over time, I eventually stopped caring so much for it.
The way I used my device had to be adjusted, but it was nothing so major. However, if there is one thing that is annoying about lack of D-Pad, it is that many J2ME apps/games simply didn't work well without it. I got over that because many J2ME apps/games didn't work well with a VGA/WVGA screen either. Still, it was a rather significant blow.
Would I like a D-Pad on all my devices? Yes, but I no longer view it as necessary.