The last day or two quite a few threads were opened on that very subject. For me personally, I'm somewhat neutral to whether I'll keep playing with it or not. I'm a skilled player with a 1000hrs in, so I welcome a difficulty hike but there are some things that will get on your nerves such as not being able to hand out titles when you're over your demense limit or having a vassal that loves you in the malcontent faction. From what I've seen it tends to be that if you start late, you'll have some if not all of the Council laws enabled which means you'll have a time and a half trying to do even the simplest of things. If you start in 769 or 867, then your laws will probably be entirely in your favor- you can then set up all of the other laws you want (centralization, vassal obligations) and then when you're a larger kingdom / small empire you let your vassals/council have a little power so they're less prone to be rebellious. So you as might imagine, Conclave sort of adds a meta-game where you'll be fighting back and forth with your vassals and you'll see some large empires like the HRE or ERE have some issues because of this (since as its always been but now even more so, more vassals equals more effort to keep them happy).
The other main feature besides the Council-related mechanics is new education system, which is more involved than the old one but is a bit tedious since you're asked to choose one childhood foci and one teenage foci for every kid in your court. It's also more RNG and event based, which basically translates into no more super heirs with a 12-15+ in every stat that you educated yourself.
Probably the best advice I could give you though is at least play with the 2.5 patch, since Conclave will be useless to you if you want to roll back to 2.4.5. A majority of bad Steam reviews were complaints aimed at the patch features, such as the new threat and coalition system (which was nerfed and is really only a concern if you're an empire or if you're spamming holy wars) or shattered retreats (which were also toned down so armies only retreat a few counties away). So pretty much if you're good with the patch features then Conclave should be a buy as long as you don't mind a difficulty hike.