What a question... OF COURSE it would've been good!
From all the things I'd read, the tech demo I played, and all the released source code that came out after the entire project was scrapped, I can say without a shadow of a doubt, Van Buren FO3 would have been UTTERLY AMAZING. As for the combat, if it had been JUST the same as what FOT presented, then it would ALSO have been fantastic. The combat in FOT had some issues, but patches worked most of those out, and left it in a state that rivals the best in its turn-based variant, with an exceptional pseudo-real-time mode that most stick to by default that works just as well if not much better, depending on the combat scenario. I always wished for and fantasized about FO2 content and gameplay to be lifted and transplanted into FOT's engine, so the mere thought of FO3 being like that means it would have been truly great.
The story, likewise, had a much more grandiose scale than the resulting FO3 or FONV titles could ever dream of. Sure, Ulysses' aims and objectives would have major repercussions for the region the game inhabits, and indirectly affect much of the rest of the country as a result, but Presper's goals, like the Master and Enclave before him, had global consequences. Stopping Presper was, like the previous games, tantamount to saving the world, unlike FO3's endgame saving a city, or FONV's saving a post-war nation.
Furthermore, I can't imagine that the 3D graphics would have grown any more dated than the 2D graphics of the originals. After all, Quake doesn't look worse than Duke Nukem 3D, and Doom doesn't looks better than the former in any way, despite all of them clearly looking dated by today's standards. Conversely, it's much easier to update polygons for higher resolution than is it to do so with pixels.
So, in short, yes, Van Buren would have been WONDERFUL had it been finished and released.