I really like this inventory mod. It's intuitive, feature rich, and everything is accessible as soon as you open your inventory. My favorite feature is the waypoints system and the ability to visualize the area around a waypoint. It also has a very nice look to it. I also like being able to see my character when I open my inventory. My only criticism, and this is mentioned in the mod description, is the 9-slots-wide inventory as opposed to the Minetest standard 8-slots-wide inventory. When you open a chest, some items in the bottom-right of your i3 inventory grid are not accessible. To access these items, you have to move them up and to the left in your inventory. This extra column of slots also means that when you open a chest, your items shift to the right one slot on each row. If you keep your i3 inventory visually organized, prepare for that organization to break during the time when you open a chest. Overall, it's not a major inconvenience once you figure out how the items shift and which item slots are not visible from a chest view. EDIT: As noted in a reply below, the inventory view can be adjusted.
The current version has the option for each player to switch to the 4 times 8 inventory size :-)
Settings -> Style -> Legacy inventory -- remember to empty your last four slots before setting a tick there!