Currently playing a monarch marathon game, and vikings are just incredible. Sure, the dutch UU would be awesome to have right now, but aggressive + berserkers is just unstoppable. Financial gave me astronomy before anyone else, and early wars for 3 great generals + civics bonuses means that berserkers come out with 3 or 4 upgrades on top of their free amphibious and +10% city attack means that I just sail around a coast loaded up on berserkers and nothing else, and drop "berserker bombs" on any city with 3 or less defenders.
With city attack and anti-archery upgrades, they've been steamrolling through super city defense longbowmen like butter, losing maybe one berserker per city I've been capturing. Like Sun Tzu says, you can't be strong everywhere at once, and the berserkers cut out that critical round in which you drop your stack next to an enemy city, and the enemy rushes defensive units by road into the city to reinforce. I've never conquered an island nation as fast as I have with berserkers.
After you capture a city, you can either dump city defense longbowmen into it, or just raze it and move on to the next. There's no counter unit to berserkers at this time period -- berserkers get +50% vs macemen, and longbowmen just have too low a strength to hold out against the viking onslaught, even with full city culture defense. I only ignore cities with huge stacks of defenders in it.
During the time period around where macemen are the top unit, the vikings completely dominate coastal cities. Later on, you mass produce bersekers then upgrade them into riflemen or grenadiers, and you still carry on the proud tradition of being able to carry out unstoppable naval invasions directly from the sea.
The UB of the vikings + financial means that I can win circumnavigation, and run around the whole game with +2 movement on all ships. While dutch ships are undoubtedly superior during its timeframe, +2 to movement during the whole game is just too awesome, especially since you're playing for a conquest victory with the aggressive vikings.