Is there any comparison between k2 and other workflows?

I don't think there is a comprehensive comparison report that is out there.

However, a quick search on Google turned up a couple of references.

http://bagofspanners.wordpress.com/2009/10/21/what-do-k2-blackpearl-and-k2-blackpoint-give-you-over-sharepoint-workflow/

http://www.k2distillery.com/2009/02/k2-blackpearl-and-metastorm-comparison.html

However, I think what K2 excels in is (my personal view):

• 
  
• Strong product roadmap tie in with Microsoft roadmap
  
• Strong community presence
  
• K2 works closely with the customer to ensure that they are successful in implementing the solution and not just buying a product.
  
• Strong product vision (I still think K2 has pretty funky technology that allows you to do some pretty amazing things with little or no code).
  

Other people might have different views but this is just my 2 cents.