I have used both... Nintex and K2 Black Point. I have yet to meet anyone, other than consultants who are K2 partners, who prefers K2 over Nintex. K2 BlackPoint (the SharePoint version, not BlackPearl) is horrible to work with. It has its own infrastructure (meaning its own server) and we lost dozens of workflows into la la land. Because it's NOT using the SP workflow engine, but rather its own, it's next to impossible to get anything that is lost. K2 support told us we were SOL and there was no way to retrieve the lost items. It caused escalations internally all the way up to our C level, because the workflows that were lost were new hire boardings for executives. The only reason the company bought it was because it was dirt cheap and the MS partner they used pushed it on them as the best. I honestly believe BlackPoint was a crappy afterthought by K2 to compete with Nintex as they are clearly the market leader in the SP workflow space.
Plus, the Designer is horrendous. The learning curve was ridiculous beyond simple serial approvals even for our senior developers. Nintex, however, is equally as powerful, uses the SharePoint workflow engine, and is so easy that non technical staff can learn it quickly. You get what you pay for... If I want to spend tons of time learning something, I'll just open Visual Studio and code the whole thing by hand using Workflow Foundations. The ONLY cool thing I remember from BlackPoint is the ability to create a Word document and populate it with data from within the workflow. Other than that, don't be swayed by the price tag...you will regret it.