My predecessor found Cartalyst, so I can't take credit for the sound decision to subscribe.
We've had a multi-developer subscription for about a year and I must agree that it's worth every penny. I'm not in any way affiliated; I'm just a very satisfied subscriber.
In our particular case, the most compelling arguments to be made in its favor are:
1.) The product is excellent.
I've learned more about Laravel from reading Cartalyst's code than I have from reading any book or following any tutorial. The code is as well-written as any I've seen. Many large, popular products have horrible, unsightly code hidden under the hood. After nearly a year with Cartalyst, I haven't seen a single line of code that made me shake my head.
I have the most experience with Platform and Sentinel (the non-FOSS successor to Sentry), which I consider to be Cartalyst's "flagship" packages. These two packages, coupled with the auxiliary components that contribute to their make-up, have thus far been sufficient for all of our application development needs (and we are a sizable corporate enterprise). These two packages alone provide content management, robust authentication, role-based authorization, a theme engine with asset-queuing and complete template and asset inheritance (with fallbacks), and more. Extremely powerful.
2.) The entire product-line is designed with extensibility at the forefront.
It's of utmost importance that we're able to override functionality to suit our specific needs. Platform is especially powerful in this regard because of its Extension system. Platform provides a GUI-driven tool for spawning new Extensions, which makes them effortless to create and configure. Each Extension is a mini-Laravel installation, which is portable, self-contained, and features dependency-management (one Extension can require another, for example), and allows the developer to override quite literally any aspect of the base application's behavior -- including logic, templates, assets, localization strings... they've thought of everything.
3.) The support is top-notch. Cartalyst is extremely responsive to support inquiries, bugs/issues, and Pull Requests on GitHub.
Support is not limited to clarifying finer points of the documentation; they never let themselves off easy. If the team can't solve your problem off-the-cuff (and they usually do), the developers will roll-up their sleeves and dig into your code, which is unprecedented in my experience. Even when it's "operator error", Cartalyst will show you where you've gone wrong, make the necessary changes, and return your code in working condition. Now that's support.
As somebody who has opened many bug reports of varying severity, I've been rather impressed with Cartalyst's ability to fix issues and tag the corresponding releases quickly.
4.) Cartalyst worries about semantic versioning, release tagging, release cycles, etc., so I don't have to.
Anybody who has ever had to manage a large, non-private code-base knows all too well the monumental effort required. Laravel's refusal to follow semantic versioning thus far has made the job even more cumbersome, yet the Cartalyst team ensures that its code is released in lockstep with changes in Laravel and that there are no "upstream surprises". Offloading this responsibility to Cartalyst has been worth the cost alone.
5.) The cost is incredibly reasonable and effortless to justify to management.
Cartalyst should (and probably will) charge more in the future. I have to assume that the current rate is "introductory" and designed to build the user-base. Even if the cost doubled, we wouldn't hesitate.
Given that the subscription costs as little as an hour of one's billable time each month, the support alone will pay for the subscription in a matter of days. It's all too easy to waste several non-billable hours struggling with some snag that the Cartalyst team would be able to address quickly and expertly.
6.) The private Gitter chat channel.
I've learned as much from the Gitter chat channel as I have from any other learning aid. There is a social aspect to the chat channel that makes learning Cartalyst fun. There is a sense of companionship, camaraderie, and mutual best-interest that fosters creativity and productivity. Inevitably, somebody in that chat channel has the answer (and oftentimes it's another subscriber, not even a Cartalyst staff-member).
7.) The license is fair and reasonable.
The ability to continue operating websites that are built upon Cartalyst components in the event that we terminate the relationship for any reason is imperative to our company. The licensing terms allow for this and are entirely reasonable in every other respect. Many organizations take this ability for granted when dealing with proprietary software licenses.
All of that said, the other assessments in this thread are fair and accurate. The documentation is good, but it does lag behind the code-base. But, I would rather have mature code with decent documentation than young, poorly-vetted code with exceptional documentation. My understanding is that they are fully-aware of the need for more tutorials, more walk-throughs, and more examples in the manual, and are working diligently to meet that need.
Knowledge worth acquiring is difficult to obtain. The learning curve is quite steep, and as davernz noted, trying to learn Laravel and Cartalyst's packages at the same time is a considerable undertaking. As someone who did precisely that, it took me several months of 8-hour days to feel like I could more or less build anything I might need (and I'm a senior developer with over 10 years of full-time development experience, mostly in PHP). But once your skills are dialed-in, the sky is the limit.