What does Zendesk Sell do well? Intuitive for businesses where the out of the box software is already exactly how they require it to be.
What does Zendesk Sell not do well? Horrendously rigid for this day and age. It's near impossible to make any record look the way you want it. You have very few different record types and they will look how ZS wants them to look. No custom records of any type.
What do you wish you had known about Zendesk Sell before using? That is was all contact centric. Contact records, company contact record, lead contact.. not a good way to layout records versus a company record with other types of records tied to it.
Have you used a competitor to Zendesk Sell? If so, how does it compare? Salesforce, Hubspot and Netsuite are all better options if you have any CRM knowledge and can make basic changes on your own.