Result: We always reached the limits of SharePoint Designer custom workflows and used tweaks to achieve out goal, but that was and is time consuming and often a fragile solution. With HarePoint Workflow Externsion we could build more robust and reliable solutions in a third of time we usually would have spend.
Bottom line: With HarePoint Workflow Extensions we solved a lot of required workflows that were not thinkable with the raw SharePoint Designer actions. If you like to compare the SharePoint Designer actions with the HarePoint Workflow Extension is like comparing "Edit" vs. "Microsoft Word". Beyond that HarePoint offers an outstanding and very helpful support.