I bought Slay The Spire (STS) during its Early Access in April 2018. I was instantly drawn in to the game, and it satisfied the same sort of rogue-like urges which I'd found playing FTL: Faster Than Light. It was a good time to grab it, because I've experienced all the artwork changes, gameplay tweaks, additions to the original game, balancing tweaks, new characters, new card-sets, new relics, and new map events. During Early Access the game continued improving and gaining more depth, and every update I was appreciating it more.

I'm writing this review, a couple of weeks after the Full Game (v1.0) was unveiled on Steam (Review written at end of Jan 2019). I have played the full release numerous times, so can comment on that. I'm going to try to write this review without mentioning things that I feel would spoil the game for readers. Also I'd like to add that I'm a Linux gamer, and the game has been working fine on a Linux operating system rather than Windows. (When game developers release a game cross-platform, e.g. across Windows, MacOS, Linux, then the game code is generally better quality, with less bugs, than games developed on a single platform - and everyone reaps the benefits.)

The game is a deck-building card strategy game played as a rogue-like adventure, traversing a map and using the deck of cards as your power-ups, attacks, defences, modifiers, health bonuses. As well as cards, you can pick up potions and relics during your journey. At certain points in the game, you might be able to remove unwanted cards, and there are some bad (Curse) cards as well as useful ones.

Similar to FTL, the road ahead of you contains a selection of criss-crossing paths, and you choose a start-point and then choose which forks to follow.) Over your journey you might choose to venture into Event locations, Camp locations, Shops, Treasure locations, and Monster encounters. Unlike FTL, this path is always going forwards, you can't retreat down one path and take another instead.

You start the game by choosing your character. (Some characters might need to be unlocked by progressing in the game before they are available.) The first character available is a warrior-style one, and you begin with a basic deck of cards and a single useful starting relic. Each character class in the game has cards which are only available to their class, but also a selection of general cards which are available across classes. This basic set of starter cards isn't very good, unsurprisingly.

As you succeed in battling monsters, you have the opportunity to gain additional cards, but also other items (i.e. potions and relics). As the game run is rogue-like, you are trying to make the best of what is available. For instance, you might be offered to choose one out of a selection of cards as a reward for defeating the monster, and those cards might all be useless. That is the nature of playing a rogue-like.

Much of the continued replayability of the game is the fact that every game run is different. You end up with a different set of items, face different adversities, different maps, choose different places to start, and you make strategic choices dependent on your status, and where you currently are placed in your journey. As a simple example, you might have had an unfortunate bunch of encounters and your character might be low in health. There is a fork in the road in front of you and you could go to a (random) Event location or visit a Campsite. By visiting the Campsite, you know you'll be able to rest and recover some of your health (and then potentially survive more steps in your adventure). Of course, the Random Event could turn out to be very beneficial (or not!)

I have never played any of the more typical card deck-building games (e.g. Hearthstone, Magic The Gathering, Artifact) and to be honest, those games don't really appeal to me. STS is a strategy rogue-like adventure / dungeon-crawler using cards as an interesting mechanic for building a set of attacks, defences, power-ups, etc. It is a completely different type of game to those titles.

You might notice that I've played 266+ hours of STS so far (Apr 2018-Jan 2019). This is evidence of the amount of replayability available within it. I imagine even someone who isn't as keen on the game as me will inevitably gain multiple hours of enjoyment out of it. I haven't even tried the community mods yet, and I've read that there are some great new characters, with fresh card decks, new events, and so on. I don't see this game going stale for a LONG time.

Another thing that I really appreciated about the game: It is great for 'casual play.' You play by making selections with your mouse, and the play is turn-based. This means you can play it one-handed, and this makes it good for lunch-time play (eat and play at the same time). Similarly, drink a pint and play. The strategy side is hard-as-nails, but the ease of play is very convenient. Going alongside that one-handed convenience is the fact that you can quickly quit from the game, your current status gets saved into Steam Cloud Saves, and then when you load up the game again (on the same or on a different computer), you can continue where you left off.

TL;DR:
This is an addictive, enjoyable, strategy rogue-like game using card deck-building mechanics. I thoroughly recommend it. It is very replayable. It is very well designed and balanced. It is lots of fun.