FPS games don't tend to age well.
Well, that's certainly true in the case of the multiplayer aspect, which would get dated over time, but the single player campaign in Black Ops would be worth playing years from now because of the plot and the story of it which I enjoyed, but since I already completed it (actually I did it three times - one of those times was on veteran) I probably won't care to revisit it, but if its something no one has played before its definitely worth checking out no matter how old it is.
Multiplayer will get dated because there's no plot and its basically just a random free for all killing fest. I'm not saying that's not fun, but over time the engine gets dated and you will get bored of the same guns day in and day out. It won't be able to stand up to a COD game made 10 years from now. For that matter the online servers would probably be long shut down by then anyway...
But again, the single player campaign is something that will hold value indefinitely.