What do you like best?
The visual campaign builder is amazing. It provides the ability to literally DESIGN what the process should be from the moment you acquire a contact (prospect, lead, sale... any level), through to delivery and satisfaction of the customer. Virtual hand-offs from one campaign to another are both possible and required to make the entire business-process-automation maintainable.
What do you dislike?
Of late (12/17 - 2/1), spam-bots are winning. A bot will fill out a lead form, using a legitimate email and a bogus name, creating a bogus contact record. You send a message (even a double-opt-in confirmation), but it's to a legitimate human who was subscribed by a bot. The human will flag you as a spammer. There are steps you can take to stop this, but they require additional, non-standard efforts to implement.
The tool is not hard to use but does require considerable practice and experience to use it well. Imagine a large Lego set to build a spaceship. With instructions, a 10-year-old can build it. But if all you have are the parts (design elements), then it is unlikely that the spaceship is going to turn out well. But once it's built, even a 5-year old can play with it. So you really do need help with initial implementation and design with Infusionsoft in order for your own business to fly through the universe.
Recommendations to others considering the product
Don't make the mistake of thinking you can do it all yourself. The reason Infusionsoft requires a partner to help with setup (Their kick-starter packages) is that they are more interested in your success than in having you purchase the product and fail. Used well, that kick-start is the best investment you can make with implementation. It's not like an automotive service-package that you may or may not use. Rather, it's more like putting gas, oil, and fluids into the car before you leave the lot.
What business problems are you solving with the product? What benefits have you realized?
After designing a campaign that tracked students through a 3-year program with yearly renewal, we found that our billing process had a hole, and many students never paid the next year's tuition. The Infusionsoft campaign made this visible, netting us thousands of dollars in lost revenue. The key here is that Infusionsoft is far more than just a CRM. It's business-process-automation.