I picked up Splinter Cell: Blacklist yesterday. Full disclosure, I have not touched single player. At least not more than the intro mission the game requires you to play in order to reach the game's menu (and therefore, multiplayer).
I find it a little annoying that you not only have to load into single player in order to select online matches, but also you have to load out to single player, in order to quit the game client.
But that aside, the reason I bought the game is because it brought back the Spies vs Mercs gameplay from my beloved Pandora Tomorrow. I'm sure I'll play through the single player (it looks good), but SvM was the selling feature.
So with that said... it's okay. The updated Blacklist version of Spies vs Mercs is decent, though it does seem like it's gone through a somewhat twitchy metamorphosis. It's been "Call of Dutified" as a friend of mine put it.
That's not to say it isn't fun, but it certainly has a different overall feel than I remember from my Pandora Tomorrow days. Luckily, for that reason, there is Spies vs Mercs Classic mode, which brings the mode back to its roots. Flashlights for mercs, nightvision for spies, and dark, shadowy levels.
I was disappointed to find that time and time again it took a lot longer to locate a match for SvM Classic than it did for the new 4v4 modes. Perhaps that was just a launch day experience and it will balance out over time.
Classic mode, for me, is the superior experience. I don't think it's quite perfect... perhaps we'll see some balancing tweaks in future patch notes to address some of the little issues, but I'll take slow, quiet shadow sneaking over the dimly-lit parkour-encouraged Blacklist variant any day.
Multiplayer features a leveling/unlock system, but it's pretty weak in comparison to other games. I saw no compelling reasons to play long-term and level up, and actually each time I sat down, I'd play a couple of matches and then think to myself "Eh, I'm going to go play Payday 2 now."
I think SvM is an adequate multiplayer offering, and certainly carries some nostalgia, but I'm going to need to dive into the single-player soon because SvM can't carry the title alone.