Chargify was used by our organization to create and manage subscriptions for all the products we developed. We needed an established and trusted API service because we provided specialty financial services to markets that were very new to our kind of business model, and required a much higher level of trust than typical consumer web applications.
I found customer service and documentation to be particularly helpful. There were a couple of instances where we had confusion about or misconfiguration with our subscriptions, and customer service responded right away with clear answers. The documentation is clearly written and it's not difficult to get started using the API.
APIs can be hard to get right, but I feel the Chargify API strikes the right balance between features and elegance.
I have a long wish list of features. Some of them: I'd like to import data that can be anonymized. I'd like better implementation of charge features on the sandbox which don't require a working credit card. I wish we didn't have to always set up subscriptions and could just do a single charge. I wish we had a multiple-delete feature for subscriptions and customers. I wish we didn't have to pick a credit card processor (that's a big wish though - we picked Braintree).
The dashboard can be really confusing, in part because it's so feature-rich, however, constructing custom products with a custom pricing set was kind of bewildering and I'm not sure we ever got it down quite right. Unfortunately, we did not have time to really test our configuration live, because it never got out of testing on our staging server. I will say that I'm really glad we never had to set up discounts for real, because that was even more confusing.
The positive impact was mostly when we actually did need subscriptions. The negative impact came when we pivoted to a more focused type of product, which did not need user subscriptions. Because we were reusing the code and had already done all the setup (and had a skeleton crew), we stayed with Chargify, but many times it got in the way. Had we continued as a business, we would have switched to just using Braintree.
If you're setting up payments for a subscription plan, Chargify is a great choice. If you don't need subscriptions, i.e. if most of your customers won't be repeat business, then you may be better off just using a direct payment provider. The limits on the number of subscriptions caused us to quickly run out of subscriptions on our smaller, budget-conscious pricing plan, and deleting those subscriptions is tedious.