I have plenty of stories from when I used to work in purchase ledger and customer service.
Klick2Contact is the biggest piece of shit I've ever used. Imagine a CRM that randomly crashes and loses your tickets, and where it can take several minutes to assign an email to yourself. Now imagine having to use software that bad whilst logging cases on a separate CRM (because K2C wasn't fit for purpose) and answering emails within a 10 minute SLA.
Best I used in terms of call centre software was Salesforce. Ironically it's not specifically designed as a CRM.
Part of the reason I got fired from my old call centre job was trying to deal with K2C.
Worst enterprise level accountancy software I used was probably a tie between JD Edwards and OpenAccounts. Best accounting software was definitely Xero.