I signed up and watched a couple pre-season games. Apparently what you get is a "live stream" of NBC Sports Boston. There is no DVR function and no replays (unlike the other NBA content on Peacock). You do seem to be able to rewind for some amount. I caught one game late and was able to go back to the beginning (maybe a half an hour earlier). The stream quality itself was fine. So I'd say not as good as YouTubeTV, which I previously used. But I'll stick with it for this year because I committed myself to ESPN, Peacock, Amazon already and don't want to add another service.
Edit: I should specify, NBC Sports Boston airs replays and so you have access to stream those live as well. Just no pre-set replay of the game itself. At least not currently. If they add that, I'll stick with it as a service beyond this year.