OK, here are impressions after 5 days or so of play. I was really disappointed at the outset and almost gave up, but the game started to grow on me the past few days.
First off, in spite of the look and feel of the combat/engine/gameplay footage, it seems like they've diverged the series from being a Diablo clone to more of a traditional mobile game clone. From a format standpoint, it's very similar to recent games like Eternity Warriors and a number of other Diablo-lite clones. Basically you have set missions to complete on a larger map, and you can repeat them ad infinitum at varying levels of difficulty. Each mission requires an energy cost to enter, and you don't get any rewards unless you complete the mission. If you die mid-mission, you can pay in diamonds to keep trying. Diamonds are the IAP currency, and there are also gold and crystals as the in-game currencies. Diamonds are used for inventory expansion, potion replacement and rolling for loot. Gold is used to upgrade your character's gear, crystals are used to upgrade your minions (more on that in a sec).
Normally I've played this type of game for a bit and then get bored or irritated very quickly, generally within a day or two. This one has surprisingly held my interest longer (so far) for a few reasons. There's actually more of an ongoing story than other games of this type in the primary solo campaign mode There are quite a few mini-animations reminiscent of D3 that take place during each mission which is a nice touch. Most the dialog is spoken, which is also rare for a mobile game. The graphics and animation also are overall top notch, again clearly influenced by D3. There's a semi-social element of adding friends; while you can't do true co-op you can bring a single friend into each solo mission to help you out. So having higher level friends with desirable attributes can be very beneficial in keeping your progress. This mechanism is also used to periodically get you free loot rolls.
There are no classes in the game; instead you choose a weapon as a primary and that kind of defines your basic combat style. You can also assign a secondary weapon to switch between, so you can easily go from melee to ranged, or ranged to magic depending on what you equip. At first it seems way too simplistic since there are no skill trees for further customization. Eventually though, you get access to 3 item based slots which define 3 active skills that you can use. Again it seems limiting at first but the more you play, the more you get different drops which have different skills, and the more you can customize your gameplay. Skills and weapons are all tied to an elemental system as well, which is used to offer bonuses against other elements. You can mix and match any weapons, skills and elements to your liking. So as an example, my guy is primary a Death-bow wielder, but I've equipped one skill that is an HP leech, another that's an AoE stun/dmg and a third that's a larger AoE DoT (all green/life element). Missions usually give a recommended damage type, but in between missions I can swap anything I want out, changing to fireballs or lightning strikes or death clouds or whatever if I want. What's the catch? Every piece of gear is separately upgradeable through a standard Fusion (and then Evolution) mechanism, i.e. sacrificing other item drops or special drops to slowly improve gear over time. This is good and bad, of course. The good is that you are pretty much infinitely customizable - but only as long as you are willing to infinitely play/grind.
There's also a "PvP" mechanism in place. But it's not true PvP as much as a Clash of Clans (not sure if that's the right example) or maybe more accurately Mighty Quest for Epic Loot invasion mechanism. You have a castle which generates currency over time which is protected by "minions" - which are also tied to the same loot/card drop system. Kind of like some older games like Dragonvale, you can tap the minions over time to get currency out of them. They too are upgradeable through Fusion/Evolution, but using the crystals as currency. Invading other players' castles is on a separate Energy system, and the more you play the better the rewards, plus you can bump up in "Leagues" for improved bonuses and drops. I'm not sure how it plays out later on, but early on it is clearly geared for the attacker to win and be rewarded, so it plays out more like a secondary PvE mode, i.e. there's not much to risk losing and the rewards are really good (no exp but cash and drops). It's definitely much easier to get the Fusion drops you need through this mode than it is repeating standard missions; I suspect they might adjust that eventually.
In any case, I think this is the kind of game that you will either hate or get bored with very quickly, or will somehow sink into your psyche and make you obsessively want to keep playing. There's a lot of micromanagement in terms of making choices on what to upgrade, etc. and there's always that little dangling hook of getting a good drop. Some might not like that but personally I enjoy it. But definitely it's moved far away from the more Diablo-like loot and level type of game, so if you are expecting that you are bound to be disappointed. I've had no issues with the energy system; usually by the time I'm getting close to running out I hit another level and it resets. I suspect that won't always be the case though.
Regarding IAP, it doesn't appear necessary at all to keep playing the game (you earn 4 diamonds for each campaign mission you complete, but only for the first completion). Rolling on a guaranteed Rare item takes 50 diamonds, and it costs 10 for each upgrade of your inventory. In other words as long as you play judiciously eventually you are guaranteed at least one or two rare items. The fusion and evolution system also keeps things progressing fairly well in that regard as well, again it's just a matter of playing/grinding the missions multiple times. The good news is some of the missions are genuinely fun with a D3-esque feel to them, and there is quite a bit of variation in terms of different maps (unlike other games in this genre). So while some may argue that the game requires "grinding" it's really not that different from D3, which eventually boils down to grinding as well, albeit in a more randomized fashion.
Overall, I'd say this is one of the better/best games of this type. It's not what I had hoped for in terms of being more of a D3 clone, but it's a strong blend of ARPG conventions mixed with more recently popular mobile/CCG/building elements.