Pros: expanded demon compendium and improved demon fusion mechanics | nice stat allocation system and interesting system of learning new skills | great soundtrack and art movement | challenging difficulty, but the option for an easier mode
Cons: may be challenging to the point of insulting for some beginners | very slow moving plot | forgettable characters | uninteresting map design and quests have little direction
I've finally poured 60+ hours into this game, so I feel qualified to talk about it.
The combat system is what you've come to expect from a Shin Megami Tensei game. Enemies have certain weaknesses. If you hit those weaknesses, you'll gain a turn and your party member may "smirk" (which gives them boosted stats for their next turn). It's very simple, but somewhat hard to master (my roommate died in a tutorial battle, in the first hour of gameplay).
Once a demon hits their peak (after they learn all of the skills they can), they "whisper" their skills to the protagonist. These skills stack - so if you already have Dia (early healing spell), Dia will become Dia+1. This allows for a massive amount of customization - you can make your protagonist a powerful physical fighter by allocating points to the strength stat and whispering physical attack, a powerful elemental attacker, your main healer or support party member, or a combination of all.
On the other hand, the plot was pretty forgettable and slow. The characters were fairly predictable and flat and the endings are exactly what you would expect from a SMT game. Much like Pokemon X or any other monster collecting game, the plot is almost purely incidental - it's as if the plot was added as an afterthought to the demon negotiation, collection, and fusion system.
To make the flat characters and plot worse, the map design is drab and a little annoying. The world-map navigation suffers from too frequent encounters and a lack of direction. There are literally no guide markers or really any landmarks to help you navigate. Quests consist of mostly backtracking and stumbling around destroyed Tokyo, hopping that you'll eventually figure out which bombed out building you need to enter.
It's not really a complaint, but it's really hilarious how easily some of the DLC can break the game. For example, for a small fee you can download three quests that practically hand you macca, experience, and app points (basically perks for demon negotiation, etc). In less than an hour you can easily amass over a million macca. Just to see how long it would take me to really break my game, I grinded for experience in one of the quests and began to fuse demons. In less than two hours I had hit the level cap (lv. 99) and fused some truly horrific demons. I then proceeded to wreck the game with my overpowered protagonist and party members. I still haven't bought the DLC armours but I'm assuming they're just as broken as the quests. However, I do look forward to the challenge quests against some of the most difficult demons in the game.
So, TL; DR:
While this game has a lot of problems - poor map design, poor character development, uninteresting plot - I enjoyed the difficulty, the combat system, and the improved demon fusion mechanics. I found SMT IV extraordinarily addicting, enough that it easily won my time and effort over Pokemon X - a real testament to the quality of the demon fusion system.