I have never had an Ancestry subscription (but I have subscribed to Fold3, which Ancestry owns -- see below). An Ancestry subscription gets you two things, so I will consider them separately:
1. Access to their record collections.
2. Ability to store your own tree, to view trees of others, and to get research leads generated from those other trees.
The prices listed right now for US subscriptions that get you both items above are either $99 or $149 for 6 months, i.e., almost $200 or $300 per year. I do occasionally use Ancestry for free at my public library, and I must say, I don't think I would find $200 to $300 per year value for accessing the records.
I have a subscription to NEHGS (the New England Historic Genealogical Society). Individual membership is $95, but I started with a sale price and seem to have been able to arrange that when I renewed. Once I found the "catalog" section of Family Search, and especially once I had made my first trip to my local FHC (Family History Center), the combination of these with the NEHGS subscription gives me basically all of the access I could hope for. I can't remember a trip to the library to view an Ancestry record that has produced anything substantive more than I'm already getting through NEHGS and FamilySearch. Those Ancestry records that sounded so enticing turned out to be duplicates of records I can find for free or else references to compiled indexes of questionable credibility. Plus, the $95 gets me access to the beautiful and amazing NEHGS building in Boston. I had the pleasure of spending a couple of days there, and it was just an amazing experience.
Now if your main interest is in, say, the Southern US, then maybe NEHGS wouldn't be so helpful. I don't know how much more helpful Ancestry would be compared to combining FamilySearch with, say, a Virginia Historical Society membership, but I'd guess the payoff is still not worth $200 to $300 per year.
Except: you get access to all of those Ancestry trees. If your interest includes identifying living cousins, that is very hard to do without Ancestry. A lot of information about living or recently deceased people is so hidden (rightfully so, since it concerns privacy) that it is difficult to track with standard research methods. The best source for this information is often just other living people who know the information, and a lot of it is availble to see on Ancestry trees. That must be worth the $200 to $300 a year to some people.
However, there is an alternative. Ancestry used to have, maybe still does have, a "DNA Insights" membership that used to cost only $98 per year. To get this, you need to send an Ancestry DNA kit in and then call them up to get the special membership. This only gives you access to trees -- not to records -- but on the other hand if your local library has a subscription, that will only show you records and not trees. So the two complement each other perfectly. Of course, you have to find someone willing to submit DNA and you have the one-time cost of the test. But this could be a good way to hack their system and get the most value out of Ancestry.
I have an Ancestry DNA kit, and I can see most of my DNA matches' trees out to 2nd great-grandparents with no additional subscription. I am also able to message them through Ancestry's messaging system. I must say I have been pleasantly surprised with how useful those two features have been. I don't know how much more I'd get out of paying the $98 per year to see additional people's trees -- maybe not much since so many people have DNA tests at Ancestry.
Concerning Fold3 -- I have found immense value in their images of war pension applications. Testimony from Revoluationary War pensions has twice helped me break through my own brick walls. And plenty of other documents there have given more information about people than I otherwise would have had. Some of these applications have transcriptions floating around, but if you want to see the original images, you can at Fold3. I could use the service for free at my local FHC, but I like to spend my time on those visits getting access to the FamilySearch images that are impossible to view elsewhere.