Is the HB monthly worth it?

Typically, yes.

Depends on unlocks...

if u already want and don't have DS2, then yes imo.

It depends, but I didn't regret the 3 I got so far. Just changed my subscription from month to month to 3 months and if I stay convinced I'll go for an entire year after :)

So far there's always been a few gems in it

If you want to play it in the next couple weeks then go for it. If not then wait for the Summer Sale, it starts before this month's Monthly closes purchases. So you can see what discount it'll get and decide then.

Side-note: It's 6 Steam games without DS2, 7 with it. And one Humble Original which is DRM-free and provides a Steam key if it ever goes there(at least Copoka did). Total of 8

Well, cards at best, will be worth like 10 cents.

I mean I wouldn't be surprised if it costs more on Steam than Humble.

Since you don't want it exactly now, best bet is to wait two weeks and see what's actual price on steam :P

Never unless you want the early unlock...or quality gibs to give away

for me it always has been so far

For me, yes. If you buy the newest games, then you might get them on the monthly and that will not be worth for you. If you don't buy the new games, then it's good for you. Check the older months and see if you like what you see.

Buy it if you want generally good games cheap, don't buy if you want to get some games that are not early unlocks (so they are not confirmed)

It only does if you want all or most of the games included, which it has never been the case for me. I normally get interesed in only 2-3 games from the monthly.
If this is the case for you, I recommend you to trade or buy the keys you are interesed in.
Anyway at the end its up to you, because it has also been proved that a lot of the games included end up being in a regular humble bundle months after, sometimes even in the Tier 1.

I love the Humble monthly, because I never buy games when they are first released, or at full price. So the monthly has been an awesome way for me to get a bunch of games that I had my eye on, but was waiting for the price to come down.

You can find lists of all their previous Monthly bundles. Take a look at the more recent bundles and decide if the blind unlocks seem like the sorts of things you'd play. If there's a good number of interesting looking things, then go for it. The quality level across the board is pretty good in my opinion, and often gives new indie games worth playing. For the question "Will Dark Souls 2 Scholar of the First Sin be cheaper in the Steam summer sale?" Possibly, but I doubt it. It might be ABOUT the same price, but as you say, you won't be getting an extra bunch of games.

The HB monthly is always worth it (if you can afford it).

Worst Case Scenarios (You buy it and it has nothing you want in it, or you don't buy and it has lots of games you want)
Middle Case Scenario (You buy it, it has some stuff you want, you make giveaways for the rest)
Best Case Scenarios (You buy it and it has lots you want in it, or you don't buy it and it has nothing you want in it)

No matter what you make good giveaways, get great games (if it's something you wanted), get great cv, and make someone's day who might not have been able to afford it otherwise.

Only if you are okay with them re-bundling the early reveal headliner game (and potentially others from it as well) for half the price a mere 60 days later.

See: January Monthly 2017 vs Jumbo 8 bundle (Vermintide & Jotun)

It's no secret that I will happily criticize the likes of Groupees, Indie Gala, or other crappy little bundle sites when they either take egregious missteps or habitually peddle shovelware, so as far as I am concerned the aforementioned was a real dick move by Humble. If you are selling people on a premium product (i.e. the Monthly Bundle) then you can't just turn around and re-bundle the games (especially the early reveal) so soon thereafter. No bueno.

I'm of the opinion that the monthly is worth it if you want the early unlock, and not worth it if you don't.

There's always one or two good games beyond the early reveal that make the purchase worthwhile. But if you don't like any of them, you can usually make a fairly good trade with the leftovers. (note that you can usually make a good trade for the early unlock too)

Three months in and never been disappointed.

I've been a subscriber since the beginning - Twice I've received games that I previously owned, but that's what steamgifts is for :P.
On those occasions the other games in the bundle have more then made up for it, and only once (aside from those 2 time's I already owned the game) have I not gotten a game from my wishlist for a lot cheaper then it would have been to purchase it commercially. - But again the other games in the bundle made up for it.
It has never really been a disappointment for me, and I'd recommend it to anyone - If only for the CV :P