I tried it out on the PS3, and it was pretty stark. If you've played marathon, it's like toggling high / low res graphics. They make the point that the PS3 version has 30% the number of pixels, and it's hugely visible.
Yeah, I played it on PS3 and prior to that all I had seen was Bungie's released footage (most of which probably came from PS4), and Alpha footage (obviously PS4). I was pretty shocked at the low-res jaggies for the first few minutes, until I remembered I was playing on a last-gen system.
That being said, surprisingly it's the same game, with the same world and the same load times.
Yeah, I was pleasantly surprised to find that there didn't seem to be any obvious lack of draw distance, enemies still showed up plenty far away in the open areas, and so on. And once the game's in motion, I don't notice the lower fidelity much unless I stop and inspect things closely. It's playable.
I would not in a million years want to play it on the previous generation though.
Given the choice, no. I'm glad I'm going next-gen on my initial purchase. I might pick up a 360 copy later if I can get it on the cheap, since I have a few friends who are sticking with that platform.