It depends on your niche but i'll be the first to tell you, at my peak, way back in the day, I was making xx,xxx a month and 90% my revenue of that was from subscriptions, this is BEFORE Netflix switched from CDs to streaming.

So subscriptions are the way to go but you have to make sure you create a community type feel around them, again, depends on what industry your sites are in but having a community-driven subscription service(having the premium members have special section to ask questions etc)

As noted above: if you have content unique and desirable subscription is the way to go. BUILDING that is a different kettle of fish. You have to provide enough content on the public facing side to find subscribers (give them a taste of what resides with membership) without giving away the goods at the same time.

THAT is the difficult part of subscription services. Gotta give some free before you can hook the pay.

THE REALLY HARD PART is to actually have worthwhile and valuable content for the membership to consume on a SUBSCRIPTION BASIS (ie, DAILY) or it all falls apart. Worst thing that can happen is for a member to feel cheated after spending money to subscribe and then getting "content light" for the investment: and you reap the bad mouthing in public/social media that likely results.

WORSE: Your subscription content is so cool and desirable that shady characters (which you can't keep out) will pay to come in and scrape your content and publish it free elsewhere---and this cannot be prevented.

HAVING VOICED DOOM AND GLOOM, subscription actually works very well. Particularly in niche content areas. The final problem is then deciding what your subscription term is: Week/Month/Year... etc. Finding that magic $number is the final key to the puzzle.

This worked for me fairly early on (1999-2004) for a few sites and some ka-ching was made along the way. After a period of time, however, all the content that could be generated had been generated and trying to fill in the missing gaps got to be an onerous chore.

About the only way to see a future is something like entertainment (think old Pulp Fiction Magazines) where the content came from OTHERS and is published under your Editorial Control. That still works, still has long legs---as long as your WRITERS are both capable and POPULAR---or your niche is Constantly In Flux with New Discovery and Science (think academic/military/entertainment).

Subscriptions are useful, in fact Webmasterworld has a subscriber section for discussion not found in the public facing side.

One more tool in the tool belt.

But the OP subject is "supplement Adsense with subscriptions"---I guess that depends on what you are making with Adsense---I suspect there are a LOT of webmasters out there that will say "anything I can make is better than what I get from adsense"---

On the other hand, SUBSCRIPTIONS are something YOU DO and are not reliant on a "free to play for what we pay" kind of deal.

I know it works, been there, done that, but it is WAY MORE WORK than slapping a bit of third party code into any kind of content to create an ad insert.

YMMV

Seems like a LOT of work but perhaps worth it. Thanks for the insights.