We upgraded from a balcony on the Pearl to a Penthouse for $500 per person. If NCL offers the upgrade and it reasonable, YES...GO FOR IT.
Check out the square footage for the balcony vs. the mini-suite, and see what the difference is compared to the price of the upgrade. Sometimes it's a great value like our move up to the Penthouse, sometimes it is not economically worth it.
We always hope and pray for the NCL direct upsells.