Mediocre compared to its predecessors. Just rent it. This should be a DLC for Modern Warfare 2.
Multiplier is the same as before. No reason to play this over MW2.
The co-op is nice and there are some new maps, but the maps are kind of silly - one map the teams spawn at houses across the street from each other and the whole map is just the two yards, in another map the teams spawn at a target practice area - thought the targets are indestructible, as well as almost anything else on the map. The writers should play BF:BC2 to see how this should be done. There are some fantasy game modes where weapons suddenly change, or you can gamble points, but these are simple diversions from the fact that there is nothing new in the game. After you get the trophies for those game modes you won't want to play them again. They could have been DLC for MW2. The only thing I can say is that my son loves the game because of the excitement - the same load level up banners as you progress through the ranks feeding your ego. He is much better at just constant running and gunning than I, and gets near the top of most matches. So if run and gun style with young kids is what you like than you will like multiplayer. Though my son still plays MW2 more than this.
Single player
The story is OK and simple, which means it is good for a COD game. This is a war game after all and you don't need a story, just some good settings and weapons. WAW had unique gameplay (the WWII battles were each unique and interesting), weapons (flamethrowers for one), and introduced zombies to COD. MW introduced a great leveling up concept, improved FPS game play, new weapons customization, and really changed the way war games look. MW2 introduced a great and uniques coop game style ( my favorite part of COD:MW2). Black Ops does not have the MW2 co-op (extremely disappointing), rehashes the zombies in a tedious grinding way, and has really, really dumn maps. Of all the COD games this is at the bottom. It's purpose seems to be to capitalize on the COD brand and nothing more. If you are a fan of the brand or a collector of COD games than you will definitely enjoy this, but I suggest renting it first.
Trophies
I have platimum in both WAW and MW2 and had fun though difficulty in getting them. I thought that maybe going for trophies would make BO worthwhile. The multiplayer trophies are too simple. as are the story trophies. The only challenge was the zombie trophies. However I will not be getting them. After many attempts, I read all the zombie trophy walkthroughs and videos I could tolerate and reallized how much time was required to get these is in a really boring, grinding way, in a pointless game mode, and I have more interesting things to do, like play BC Viet Nam.
Summary
I would rather play WAW, MW2, BF:BC2, MAG, or MOH again than play this. I would regret buying this game with so many better options out there, however my young son loves it, or he did until KZ3 came out. Now it just gathers dust. You will go through the story in about 2 hours and it has no replay value. The multiplayer is the same as MW2 and the MW2 co-op is gone. Rent it.