Atlas frankly mismanaged the offboard experience for a lot of passengers during their first year, but with new leadership in the second year, things have improved. One constant from what I've read is that the onboard product is excellent.
Embarkation - Surprisingly very easy, despite the late boarding time. We checked in at Terminal C at EZE and it was seamless from there to being onboard. After arriving at Ushuaia and getting luggage, we were bused directly to the ship. Atlas had agents guiding us at both airports. The actual ship check-in was in the ship lounge after boarding, where I presented my passport while sipping on a glass of sparkling wine. Very civilized. The only hiccup for some would be that the charter flight is not exactly luxury. The safety drill is more involved than your typical cruise ship drill. They actually made you wear your life vest and assemble at the muster stations, but completely understandable given that we are sailing in one of the world's most treacherous waters.
Drake Passage - Going to Antarctica, we had like 3-4 meter waves. The return had 4-6 meter waves on the first day but was a bit calmer on the second day. I didn't get seasick and even didn't take any medication, but the rest of my group did get hit with the Drake Shake. A couple of my friends were out almost the entire first two days. The helpful hint is to apply your patch and take your medicine before going to bed when you leave Ushuaia. Definitely got the feeling it was an expedition, not a cruise when your dishware and cutlery start sliding off the dining table, and when you see waves go over the deck 7 viewing platform. One expedition guide joked that getting seasick is just a tax payment from Poseidon to see the wonders of Antarctica.
Got upgraded onboard, lots of space for this horizon deluxe balcony cabin. Best bathroom at sea with the amazing shower. Ample closet space, almost double compared to standard balcony cabins. Liked that the buttons to lower the window was close to the window. Also loved that you could view the scenery while lying on the bed. Cabin temperature was customizable with a thermostat, I like to keep on the cool side but it is very easy to get warm or hot.