This game is amazing. I'm having a lot of trouble deciding which class to take with me for an entire playthrough, but at this point I've played through the first few hours of the game 5+ times and I'm not even remotely sick of it. Also surprisingly for a game like this (released by these particular developers), the game is remarkably polished in terms of bugs/glitches. Obsidian's testing team has just shamed the ever loving hell out of inXile's Wasteland 2.
The combat flows extremely well once you get used to it - enemies for the most part go for who they see first, so formation and positioning is VERY important. The stat system itself is actually done in a unique way that takes some getting used to, as well - namely that the stats do the same thing for everyone. Might, the stat that one would usually compare to Strength, for example, is necessary for any class looking to deal any kind of damage (Ranger? Wizard? Better double down on that might as if you were a Barbarian). Dexterity quickens attacks just the same as it does spell actions and reloading ranged weapons. Intelligence is mostly for AoE and skill duration, resolve/perception for defense (also very useful for conversations), and constitution for the same god damned thing it does for every game ever. Resolve probably has some of the most interesting character interactions, particularly in the tutorial when a survivor is being held hostage and if your resolve is high enough you can insistently suggest that the leader should just kill the hostage because by the time he's done with the killing blow you'll have advanced enough to kill the leader. Basically it's a good attribute to take if you want to go around with the biggest fucking balls in the world.
Every companion I've met so far is interesting in their own way and well written. Some guy was quoting some review that said that the companions were on par with anything Bioware did or the ones in Baldur's Gate (2), but weren't really comparable in quality to Planescape Torment, which is probably accurate.
The side quests are definitely involved, in fact there was one that was so well done that I was surprised at the end of it that it didn't have a single thing to do with the main storyline.
I haven't come across any choices that REALLY matter in the beginning yet (as I haven't gone far into the actual story at all yet), but at every turn the way you complete a quest is judged in terms of cruel, benevolent, clever (translation: smart ass(hole)), diplomatic, stoic, rational, or deceptive. At some point in the game people start reacting to you differently by how many points in each of these you've acquired.
So far there's only been one instance of a fight where I had to meta-game and take advantage of the games combat system, sending in my most expendable companion to initiate a conversation/fight when the other five members of my party huddled in an out of the way room. Then, I had to kite about a third of the fight at a time to my little out of the way room in order to survive. There were definitely more cowardly ways to handle the situation, though, or I could have (and probably will in the future) avoided the fight until I had a couple more levels under my belt. Considering the loot in the place more than quadrupled the amount of money I had, this is probably what I should have done.
More on how much I love this game when I eventually decide which class I want to play. They literally all (except fighters) seem fun.