Just started looking some of it over my self. Boy do I have some negative feedback...
Amazon, meh who cares.
Barbarian, might come back to later but I doubt it.
Necromancer/Sorceress, mostly spell based so see below.
Paladin
While Convention and Vengeance remains useful even in Hell mode (more so if partied) the P&P version sucks. +6d6 damage in total for vengeance and a mere -2 AC/Saves & -20% Resistance (0-20% is still 0, lame). Even with this book being printed long before patch 13 Blessed Hammer isn't even remotely powerful. Is it sad I'm looking at Thorns and thinking it's one of his better Auras? +200% return rate, take 12 damage from an attack and deal 24 back. I suppose since you'll fight a lot less creatures at once you can try chugging potions while wearing a hit me sign on your face. Honestly the Paladin sucks. Zeal caps at once attack per opponent making it worse than the feat intensive Whirlwind feat, the Meditation aura requires DC 15 Charisma checks so your average Wizard would be better off selling you into slavery. It's different, call it flavorful if you think you need to find a compliment. But nothing scales and in all it's weaker than a SpC powered D&D Paladin.
Armor
I thought I'd like the Head/Hands/Boots/Belt set up. +1 for chain boots, +1 for gloves, +3 for a bone helm, +1 for a simply sash. Custom armor, +6 AC, spiffy! Then you read the little star; "Not cumulative with armor. ... Caps, hats, gloves, leather boots, and ashes are also not cumulative with other partial armor." +3 to AC for the Bone Helm, f*ck the rest. Well who the hell cares about this so called system then? It's just a dozen +1~3 Armor entries.
Spells
Bone Armor: Why not DR like it should be?
X Armor: No Slow effect? Lame.
Lower Resist: -30 points of Resistance (to min 0), this is fine but why isn't Convention like this?
Raise skeleton: Using this spell ends the skeletal mage spell or any golem spell. The entire point of the Necromancer is to have an army. Is there anything they didn't screw up?
Magic Loot
The prefix "Skull" (like a "skull sash") gives +4 to saves. Really? Skulls give a leech HP/Mana effect in Diablo II so where did they pull this out of their ass from? Anyway, some useful effects can be found in here though. Mostly if you ignore the warning about the pricing values. Like +5 AC is 17k,+4 to saves is 18k, +5 all ability scores is only 50k.
Monsters
Fire Golem is CR 6, this is a 6th level Necromancer spell meaning you're level 12 and summoning CR 6 monsters. D&D Clerics can access this spell by the 13th level which means it should at least be comparable to Summon Monster XII. ...Which summons CR 6~9 creatures.
Overall
I really could keep going nit picking but this book is just so damn terrible. It's like the authors never even played Diablo (I or II) to begin with and had very little knowledge about D&D 3.0. I mean, you could have up to 30 Points invested in an ability AND items can get you higher, but that was flat out freaking dropped in favor of static abilities with flat rate bonuses. Many effects behave nothing like their Diablo II counterparts. The thought put into it can be showcased in the preamble on Magic Items, it acts like you are supposed to stack your boots and gloves AC bonuses together which is completely against the rules it already laid out a dozen pages before hand.
TL;DR: Diablerie is a POS. Google fan sites and forums for home brew material as it will be superior in quality and would have been made by people that played the game. Plus they will have the Assassin and Druid classes.
Good luck TML.