Some tools are so intuitive and ripe with potential that they are actually a delight to use. This is one such tool. In addition to being able to create databases quickly, the ability to create apps to manipulate and view records is easy and useful.
The app builder is my favorite feature. There are a lot of useful views to be used (search, forms and tables are my top 3) and I love it when an end-user asks me if it can do XY or Z and I'm able to come back with a solution quickly!
The automated tasks are also proving to be helpful for resetting values after a defined amount of time.
I haven't found anything to dislike but I don't feel like we're doing anything with Knack that would push it to its limits.
We had a great experience with Knack's developer support while building a custom solution. Don't hesitate to reach out to them if you get stuck. Be sure to check out the knowledgebase as well. They appear to do a good job keeping it fresh.
We're using Knack to track transactions on our website and then offer promotions based on the user's order history. We've also built a view that allows our customer service team to look up transaction histories and make adjustments, if needed.
By using Knack as a part of this solution, we've saved time by quickly building the interface for the app with Knack versus rolling our own.