I wouldn't pay it. The channel count is high and their brands are well-known, but the content doesn't look all that attractive to me. If DC Universe and Warner Brothers end up amounting to something, I'd probably pay around $8 to get them along with TNT. HBO and Cinemax are just too seasonal (and have too much overlap) to warrant a full-time subscription in my mind.
Boomerang used to be a bonus but I wonder anymore.
Ha. What would/do you actually spend money on, in terms of TV? For my tastes, HBO Max will have a lot more hours of quality content than Netflix 4K for the same price. At $16, I no longer keep Netflix year-round. There are some gems on there but they're floating in a sea of mediocre, unfamiliar stuff. And Netflix has become a pretty barren source for movies (other than Netflix's own original films). I would keep HBO Max for most of the year, maybe year-round.
HBO and Cinemax already have a combined total US subscriber count of about 55 million. Only a fraction of those subscribe to both; my educated guesstimate is that the number with HBO is around 40 million. HBO by itself (whether as a standalone streaming subscription or an add-on to a cable channel bundle) currently costs $15. (Actually, a few MPVDs charge more, as much as $18.) Cinemax typically costs $10-12. So, as the numbers show, HBO and Cinemax are already doing quite well, surpassed only by Netflix, at about 65 million US subs (and likely approaching peak saturation here).
Combining HBO and Cinemax, along with a whole lot of additional ad-free content that appeals to broader tastes/ages/demographics, for just another $1-2 more than HBO alone costs now should result in some significant subscriber gains (especially among non-cable TV subscribers who aren't otherwise able to access any of the current basic cable content from TNT, TBS, Cartoon Network, etc.). I don't see how this service -- along with the introductions of Disney+ and Apple TV+, as well as the growth of Hulu, don't start eating into Netflix's US subscriber numbers.