I've always enjoyed my time with Path Of Exile, but I also always felt that it's a game better played in your mind; Figuring out builds, trying to create good characters using the skill tree planner, looking at gems on the wiki and thinking about what would work to get you to the end, and then some. Sometimes, you can also look at Builds Of The Week on their forum as well to get inspiration. In reality tho, I get bored kinda quickly, either my characters die all the time, or they can't do anything, or they're just not fun to play. Fall Of Oriath adds loads of content on top of the original PoE experience, but I couldn't see it more than once.
When before you replayed the same content three times at increasing difficulty levels, now there are five more acts, continuing the story where before you just did it again. Instead of having hard cuts where you lose resistances and start losing more experience when you die, this happens over the course of this new adventure. The new content is creative and interesting, having you fight new enemies and bosses in familiar areas, but since I already have issues with the difficulty in PoE, facing new bosses for the first time on higher difficulties where you'd lose experience when you died wasn't enriching the experience at all. There was one boss early on, a giant crab, that would just kill me instantly when I entered its arena, losing me a percentage of my total experience each time, and I couldn't do anything except get lucky.
Although the new content is cool - and that harbinger league is interesting - the middle of the game is really rough for me, and the number of characters I've made that managed to get to the new content is ridiculously low. I also haven't managed to complete the third difficulty of the labyrinth, and the map end-game is just an exercise in frustration, and losing experience. I feel that in order to hang with the higher level crowd in Path of Exile, you still need a bunch of legendary items tailored to your build. These Builds of the week I talked earlier all have their own 'required legendaries' sections. If you don't miraculously find these items, you're out of luck. While in Diablo 3 you'll always going to get items that match your class - and the endgame dishes loads of them - there is no such thing in PoE, there is a very large number of legendaries to get, most of them will be useless to you, and they are drip fed at incredibly slow rates. I'm not sure if you're "supposed" to trade for them, but in their new Solo Self Found leagues - where you can't trade and party up - it's not possible.
Fall of Oriath might be compounding on this issue, but it's not all bad. I really dug the new narrative and defensive skill system that they give you for defeating and absorbing gods. I also think that the end-game map system is great and has ton of theoretical replayability. I'm sure they added a bunch of new legendary items as well that allow for some really special builds, alongside the vast swath of buffs and nerfs on the skill tree. I managed to make it to the end of all content - using a Totem build, as I usually do - but then I started getting into severe survivability issues, just dying instantly to any and all enemies. I created a bunch of other characters as well, but none felt fun, so I stopped before getting to the map end-game with a single other character. For free, Fall of Oriath is a great content add-on, but there are still some issues that prevent PoE from being my favorite action RPG.