OK so this is my stance regarding Leviathan.
I personally own it, even though it's not a must have DLC. Features like :
Favors may come handy. Example, if you are allied with a nation and that nation has an ally that is your rival or you desire their land, you'll probably want to break their alliance so you can avoid losing ally and getting yourself in unnecessary war.
Monuments. Now this feature is fun at least for me. I'm glad that Monuments aren't just cosmetic any more but have actuall purpose.
Next one regards playing tall so I'll start with Concentrate development. In my opinion it was totally unnecessary feature. It is fun to use CD on territories, lower their cost and core them. This is followed by Pillage capital. Now this one i actually like since these things did happen. Like Venitians pillaging Constantinople from 1204 and so on. Expand infrastructure, it's cool I guess. You can basically build multiple manufacturies as long as you EI. And last one is Centralize state. Lowering the cost of state doesn't make much difference.
New missions and events added to SEA are fun I guess and it was the main thing why I buyed Leviathan.
There are more things about Leviathan but I'll skip that and ask simple question is Leviathan worth for you 20 EUR? If you plan to play in SEA, NA or Pacific it may be. Or perhaps you want to get it on discount. On the other hand we all know how Leviathan launched and reviews on Steam. I really can't say to you "Hey you should definitely get that DLC". Also most of the issues Leviathan had, bugs and glitches, are fixed in later patches and some of the new features were reworked as part of 1.32.
In the end it's really up to you.