Nobody's responding, so maybe a bump will help.  Doesn't Optimum sell a smaller size?  Like a 2.5lb or something?  Why not try that first to see if you like it.  I once asked on another discussion board if a certain brand tasted good and someone said it did.  When I got it I thought it tasted pretty bad.
With that said, I know a lot of people think Optimum nutrition's protein powders taste good.  Though I don't recall anyone speaking specifically about the Pina Colada flavor.  
Personally I take EAS Simply Whey and I think the chococlate flavor tastes good.  I'm gonna try the strawberry flavor next.