Some Beserker Rush thoughts:
~ I don't use Stacks so much as I use waves. Greater flexibility for me with several smaller "stacks" than going for a beserker SOD. With a inter-hemisphere, galley-beserker-agua-assualt, I am often using the same overworked ships to ferry troops that come in waves anyway.
~ I like trying to generate a Great General to attach to a single Veteran Beserker (with CR2 +) for all the promos and include the terrain movement bonus as one promo in addition to Command promo (faster promos) and a few first strikes in addition to make him the city breaker. Match him up with a horse archer and he becomes a true leader that can both reconnoiter, pillage, defend, and crack skulls at the front lines of the city siege. Sufficient pre-Machinery warmongering with Axes can generate Two Great Generals; one for an Academy and the next one for Hagar the Horrible.
~ While it is true that the Naval game mechanics are a bit broken, I believe it is a bit of a mistake to underestimate the power of an additional movement (or two) when it comes to defense and actually using ships in a proactive fashion. As Astronomy takes over and the oceans open up, Frigates can actually keep the homeland safe from counter invasion. A small fleet of ships can actively patrol like a coast guard while our now upgraded (!!) (and very veteran with many promotions) beserkers are CR3 Grenadiers mixed with cannons and are getting ready to run across the next continent like Patton did across Europe.
~ Amphibious promotion. Useful across rivers and also it frequently can save a movement turn or two while staging the attack. It is nice to be able to use it to siege across a river while the enemy suffers the penalty if the try to counter attack out of their city. Is it a game breaker Of course not, nor should it, or any other UU characteristic be a game breaking, panacea. It adds yet one more option, and in a game where multiple pathways are cherished, I think it is a great option to have. It is also very useful on Pangea maps to invade coastal cities. I think that the concept that only arichipleago maps suit the Vikings is not wholly correct. True, that map type feeds into their strengths, but these attributes of attacking from the sea and having additional water movement can have tremendous benefits in moving a large scale invasion fleet to another point on the same continent. It is especially fun to coordinate a simultaneous land and sea attack and the Vikings make this a very doable strategy on several map types (Great Plains?!)
~ Tech path is key. With a CS slingshot, it is really almost an exploit. It is very easy to have DOZENS of Beserkers going after cities that have only archers, spears, and axes as their best defenders. Even with your opponents grabbing Feudalism, it takes while before the AI can fully retool and get all Longbows up.
~ Without the Slingshot, the window is appreciably smaller, but with the right tech path, it is still a nice window of several hundred years before you see all your opponents cities defended with twin Longbows. I do not wait around for construction before using the Beserkers to conquer at least one or two Civ's. In fact, I think the Beserker allows me to push Construction back a while so I can go after Feudalism myself to grab an early capitulated Vassal that may have some resources. Instead of straining my pre-currency economy with baby-sitting a bunch of newly captured cities, I pick a few plums, allow the Vassal to work the one city that is only there to grab the Wine (or some such resource). Grabbing the Vassaled Civ early helps the economy because you just keep going to the well for money (exploit?).
~ More tech path. Becasue I keep Construction off for a while, I grab Math but then Currency to cash-cow whatever techs I can starting to build the warchest for the Beserker upgrades. Construction usually comes as a fill-in tech while re-organizing the REX-ed Viking Empire which takes the cue, grabbing a quick breather when Feudalistic Longbows become a sticking point. Now its time to just reload, re-aim, and get ready to refire. OR, one could now choose to become illuminated & civilized, keeping the present enormous chunk of real estate won by Beserkers and now easily defended with beefed-up Grenadiers then Rifles while aiming for some other type of victory.