I’ve been playing my DLC copy of New Super Luigi U and this is what I can tell you so far. Once you’ve updated your New Super Mario Bros U you will see a link where you can connect to eshop and purchase the Luigi U DLC.
What you need to know is that Luigi U is more like an alternative universe to Mario U as in the story is exactly the same, same cut scenes and same world map. What is different here is the stages are completely different and actually more challenging.
It takes some time to get used to Luigi’s controls as now he jumps a longer distance than Mario and you might find yourself over-jumping and landing into some turtles at first. Multiplayer is fun where someone can now play as Nabbit, Nabbit has the advantage of not getting hurt by bad guys but he can still get crushed or fall down holes. The downside to Nabbit is that he can’t eat Mushrooms or any Power Ups. The time limit on the castles can be very challenging in multi-player mode as you have to wait for other players instead of rushing.
This has so far been a great DLC add on which puts new life into your Mario on Nintendo Wii U. I find myself progressing quickly through the stages so if you would like Luigi U to last longer, then try collecting all the big coins for each stage. I haven’t gotten to the new challenges modes yet but I look forward to more Luigi game time.