While Disgaea is as you said primarily a comedy the better mainline entries do have some solid moments of pathos (I'd also question how many serious SRPG's actually manage complex and/or mature story writing but thats a separate discussion)
I really hate their extended DLC schedules, the DLC is generally too short to make use of on its own so I like to wait for it all to come out and then play through again, but it takes so long to come out that Im seldom in the mood for another playthrough at that point (and then you also have to wonder if buying it all along or waiting for a mega DLC pack which may or may not come is a better idea). While I like to support the series they do a good job of convincing me not to buy DLC (apart from Zetta of course, because Zetta)
D5's Mega DLC pack basically is it's Season Pass. All of the content is already known for it. So it's not like some other Season Passes with mystery content that may or may not live up to it's advertised planning.
It was 4000 Yen in Japan which equates to about $35 USD. Of course there's no telling what NISA will actually price it in the west for US and EU.
If it's priced higher than the Japanese Season pass then I don't think it's worth it.
For example NISA has a history of marking up the US / EU DLC pricing to the same numbers as the Japanese DLC pricing.
Case in point 300 Yen for Disgaea D2 characters is equivalent to $2.50. NISA simply sees the 300 number and makes it $3.00 / €3.00 instead.
So if 4000 Yen in Japan becomes $40.00, we pretty much know NISA is not doing a proper price conversion is simply marking it up for the Western Market.
I don't think there is any current season pass for a non-AAA game that costs $40 anyways, so NISA really can't justify a pricing scheme like this.
But anyways as for the complete content included in the Season Pass according to the Japanese release.
Wave 1
Disgaea D2 Chapter: Laharl, Etna, Flonne, Sicily
Laharl-chan
Wave 2
Disgaea 4 Chapter: Valvatorez, Fenrich, Artina
Fuka Chapter: Fuka, Desco, Emizel
NISA
Wave 3
Disgaea 3 Chapter: Mao, Raspberyl, Salvatore
Almaz Chapter: Almaz, Sapphire
Metallica (The Witch and the Hundred Knight)
Wave 4
Disgaea 2 Chapter: Adell, Rozalin, Axel
Wave 5
Makai Kingdom Chapter: Zetta, Pram, Peta
La Pucelle Demon Overlord Priere Chapter: Demon Overlord Priere
Each of the "Chapters" are mini-episodes that consist of 3 battles each and the there is
Pleinair, Toro, and, Kuro were bonus additions, but I don't think the Playstation Cat mascots are going to be released in the west. There was some PS Plus related content, three extra classes, Female Ninja, Female Angel, and Sorceress, but these are not PS Plus for the West apparently -
http://blog.eu.playstation.com/2015/08/06/new-disgaea-5-features-detailed-dlc-schedule-confirmed/
So to summarize.
8 Mini Chapters of 3 Maps / Battles Each, with 26 Characters, and 3 Bonus Classes (28 Characters IF Toro and Kuro are released in the West), It's a fair amount of content, but I hope NISA is reasonable with their pricing scheme. If not, I'll be waiting for a discount.
Also I think it should be fair to mention the DLC Chapters and Characters are usable immediately upon buying them. They are not delegated to Post-Game only content like in the previous games, so the content you do buy you have access to right away the next time you load your save.