Actually I watched both of them last night. I figured it would be a bad idea to see the sequel without having seen the original, so I rented it and watched it earlier in the evening.
Turns out I had seen the original before, just forgotten about it. Which is odd, because it's a good film. Sure it has some goofy scenes and characters, but for the most part I loved it.
And just an hour after I finished the first film, I was on my way to the theatre to see the sequel.
Since we saw him last time, McCall (Denzel Washington) quit his job to start a new career as a Lyft driver, presumably to avoid situations like in the first film where his action would get his co-workers in trouble. Because he still does those actions; occasionally walking into a room and killing everyone inside it to avenge some wrong they have done. This time it's not his local crime-fighting hobby which gets him in trouble though, but him getting involved in the investigation of the murder of one of his close friends. From there he stumbles onto a group of ruthless assassins, and you can see where this is going. Except this time, since he has no co-workers, it's his innocent neighbours that get dragged into the action.
This film also reveals more of McCall's back-story (including that McCall is apparently his real name, which is odd since we know he faked his death to get out clean of his special forces career). We meet more of his old friends from the CIA, get to visit his old home town, and get to learn a lot more about his deceased wife. This all makes for some emotional moments, but the first film showed that knowing his back-story isn't actually necessary to thoroughly enjoy watching him murder bad guys with brutal efficiency.
The action scenes are for the most part great, especially the first one (a walk-into-the-room-and-kill-everyone scene) and the climax (where he's faced up against a heavily armed strike team in a small town that's been evacuated from a storm). The first one sets up some false hopes though, since the highlight of the first film for me was the scenes where he would carefully analyse a situation, sometimes even going as far as moving around some small objects in the room, before slaying everyone in a manner of seconds. Yes, very similar to the fight scenes in Sherlock, just missing Robert Downey Jr.'s inner monologue.
Another slight disappointment is the lack of characters from the first film. I felt like we really got to know Teri and Ralphie, but in the sequel they are never seen nor even mentioned. Instead McCall takes a neighbour kid under his wings and tries to stop him from getting involved in the local street gangs. Fair enough I guess, since they wouldn't want to have so many characters in the story that it gets confusing.
A bigger disappointment is how frequently McCall uses guns in this film, something he barely touched in the original. There part of the fun was the many inventive ways he was able to take out an opponent using everyday items.
All in all I felt the sequel was slightly weaker than the original, but still well worth seeing in the theatre. If you haven't seen the first one, I wouldn't worry about it, since so few of the old characters show up, and the events in the previous film aren't mentioned at all as far as I can recall.