I wouldn't cruise without one. It is so relaxing to sit in private overlooking the ocean, drinking your morning cup of coffee - and fun watching docking into the next port without jockeying for a prime spot on the public decks. You'll miss a lot on an Alaska cruise if you don't have a balconey (reserve a starboard cabin). Bring warm clothing and you'll be happy sitting there. Balconies are also a must for a Caribbean cruise because you will want to wear virtually no clothing and no one can see you on your private balconey!
Beware privacy on some of Princesses balconies. Their decks are stacked on some ships, so the people above you can look down on you - not very private.
If you can afford to spend a few hundred extra, it is more than worth it.