I have not been on Adventure before. I have sailed two Voyager class ships before.
Mariner I have sailed on more than any other ship, close to 50 days onboard. I love that ship. It shows its age here and there but it has been amped and has many wonderful features added to it such as Playmaker's, even though the venue isn't really big enough.
Explorer I sailed once. I was honestly not a fan. The first three ships in the class had a slightly different design and while it is possible I was just used to things being laid out a certain way and perceived it negatively simply because it was not, I felt like the layout was clunky and they made great improvements on the later ships in the class. It was fresh out of dry dock (third sailing post) but it seemed very worn down and tired. It was not amped and did not receive any of the features present on Mariner or other new ships.
The AC was not working well in most areas of the ship, including my stateroom. A talk with Guest Services, who acted like they did not know there was an obvious problem with the AC throughout the ship, did resolve the problem but by the end of the week my room was running hot again so whatever the did was a temporary fix.
The internet was upgraded to starlink, but it did not matter as the ship was not capable of handling the speeds starlink was capable of. I had the slowest speeds I've ever had on an RCL ship, including before starlink, and had no service in my stateroom or the dining room of all places. It was just all around a ship that was past its prime. I could not walk from one part of the ship to another without having to reconnect to the wifi.
Adventure will be more likely to be like Explorer than Mariner as a first generation Voyager ship. That said, for the right price or itinerary, I would still book it, which is exactly what I did 🙂
Just be aware that it may not have the same level of upkeep as the newer classes of ships. Any smaller than Voyager class I would have to really be in love with where it is going, but Voyager class is still an easy sell, especially out of Port Canaveral.
If you have sailed on Freedom class, it is basically a slightly smaller version of that. There are less people, but the venues are mostly the same size. Same number of elevators. The biggest difference is Sorrento's has it's own location which eases the crowds at Cafe Promenade slightly and there is a kids splash pad which does nothing for me personally as an inside person.