Possibly one of the best action game I have ever played.
First, before even getting into combat there are a lot of customization for the mech (an "armored core" or "AC"), which can drastically alter it's behavior and stats, allowing the player to choose their own playstyle (whether it's actually viable or not is a different matter). For PVE, most build will be able to clear the campagin, depending on skill level. Furthermore, there is often no clear "upgrade", as in this components are strictly better than its counter part, all of them get trade off. For example, a bigger gun will have more damage but also significantly heavier, affecting other functions of the AC.
The game is set up as a series of short missions, which can vary in length, at like 10 or 15 minutes max. For me, this is quite addictive as I more often than not keep saying "just one more mission" and then end up playing for more than an hour. This also makes testing build very convenient, and the game allows for changing build mid-mission at checkpoints if you fails.
The mechanic during combat is also simple, not very complex control. An AC feel fast and powerful, it feels very different from just another third person shooter but with a mech skin, it does feel like controlling a machine. The combat is typically very fast, but a rythym is still required with reload, heat managements for weapons (there can be four firing simultaneously at the same time), coupled with energy management for the AC, which control dash, fly,.... Therefore, it doesn't feel like mindless button mashing (which can technically still work with certain builds and levels) and does take some skills to do well, which is very sastifying.
