The title pretty much says it, what's your opinion on Infants being a life-stage brought into the game. With or without Growing Together installed.

My first experience with them was with Growing Together, which I bought because I heard that the EP was focused heavily on expanding on this new life-stage. It was actually an EP that brought me back into playing Sims 4 again, after having not played since around Cottage Living/Snowy Escape. I rather quickly stopped playing again, but gave it another try eventually. Overall, I want to say that I find the Infant life-stag rather cute and sweet, with stuff like giving them naps in your arms.

Without Growing Together installed, I will say that I find Infants a very boring, if not 'useless', because there's nothing for them to do. They can instantly sit up and crawl, which works fine for things like mobility and they don't have to be carried everywhere, except for their cribs or high chairs for food. I'd overall say they are kinda 'worse' than Babies (or 'Newborns' as they now differentiate between them), as at least I don't have to worry or pay too much attention to them.

Growing Together does make Infants a lot more involved in playing, as they start off as being not able to do anything. Only lying on a rug or playmat, having to interact with the environment to learn from just reaching to finally attempting to stand up. It does require more work, and it does lead to them being completely helpless. They have to be carried everywhere, until they manage to unlock the Milestone of Sit Up and Scoot. But it makes them more interesting.
I rarely have Infants getting cribs, though. I tend to keep them on the playmat for most of the time because they can work on the finer moto skills on it, fall asleep on it, and get Tummy Time with either a guardian/parent or on their own. Just no reason to take them out of the crib, and then eventually take them out to put them back on the mat.

In fact, unlocking Milestones are a fun part of the game.
Though I personally found that, upon the Sit Up Milestone reached, trying to feed the infants in a high hair with either 'first food' being mashed up, soft food or even tiny treats referred to as 'finger food'. Even if they turn out to like the food, it feels like it doesn't fill them up as well or as fast as bottle feed/breastfeed does. I almost have to try going down a good half of the list to get them adequately fed.
The Quirks that can be unlocked are neat, as they can lead to even twin infants or just single infants being different from the next one. Both might have the Cautious trait, but one having the Self-Soother quirk means they're 'easier' to get to sleep.

At the same time, I want to say that Growing Together makes the Infants more involved, which I also find a negative for the sake of playing more in that household. After I finally got the Infant to adequate enough independency in terms of sitting up and eating, they turn into Toddlers. And then I have to go back to working and babysitting them to work on their skills, with them being just as involved in taking care of them. Hence, I feel like I am working on an extended, over-reliant-on-me Infant.
Small note, but Toddlers having Milestones for stuff like 'walking up stairs' don't unlock, despite Movement Level 2 saying they can climb stairs now. Apparently, it kinda wants you to be at Level 5 Movement, instead.

I think I can cut down my opinion on Infants being that I like their addition, but they really are only 'good' to have, if you have Growing Together installed. I think I wouldn't mind Infants so much, if there was an option to skip the Infant life-stage, if Growing Together was not an installed (or 'selected' in the Pack Selection menu).

Annoying little shits.

Pain in the EAss. Only worth having in a single parent environment where there are absolutely no other sims who are all going to start tripping over each other and getting in each others ways blocking each other all rushing to tend to infants/toddlers. Been the same since Sims 2. Somebody needs to take all sims devs and forcibly sit them down to watch that Russell Crowe movie Beautiful Mind to finally clue them in regarding the Nobel Prize winning mathematics systems to avoid cock-blocking and all variants off it where if everyone rushes to do same thing at same time then nobody can get it done. Then, maybe, they'll finally get it right. Maybe.

But the infants themselves, yeah, cute as all out.

That means something completely different where I live!

What, it doesn't mean the alpha male chicken gets to be the one who crows loudest in the farm? Only kidding. I think it means the same thing everywhere, something a group of lads might agree upon avoiding if heading off to a nightclub together in search of each finding themselves a... date. But it is so very succinct and apt a term in English that it should apply to a lot more than just a bunch of blokes on the cop in nightclubs. After all, in that very same Russell Crowe movie, that is the very scene depicted, - the men at the bar getting in each others way going after same woman - that was the inception for the Nobel Prize winning mathematics. I'm just going with the flow, here.