Acuity helps us offer a variety of services to our clients, who can easily schedule online. Compared to the other systems we tried (roughly 2-3 others), we found it to offer more features (and better support) for the price.
There is no perfect system and it's hard to find things to criticize here, but I'd like to see a more graphic interface (e.g., with more of a card view than a drop-down view) so services can be featured more prominently.
I'd love to have greater flexibility in terms of classes and events (and how these are paid for).
It would be great to offer integrated CRM and e-mail marketing functions so that clients who schedule through the system can receive follow-up promotions, birthday greetings, and coupons, etc. (And maybe they have this and I just don't know about it).
In the case of Schedulicity, the system was awesome (very easy to set up and had some nice features) but did not offer the functionality and flexibility that AcuityScheduling does. In the case of SimplyBook.me, they offer lots of functionality but the interface is extremely complex and you pay a la carte for features so you're going to pay exponentially more for the same set of features as compared to AcuityScheduling. Their customer service was also lacking, in my opinion.
I honestly can't think of a situation where the system would be ill-suited to anyone. It's more appropriate for those who have more than one service to sell, I suppose. And it might not be as helpful when you're selling products (vs. services), so that would be one thing to improve.