Recent reviews by Superb Owl

Calling this game a 2d Dark Souls, or more affectionately Bug Souls, is not far from the truth.

Hollow Knight does just about everything right:

Great zone design, beautifully composed soundtracks, tricky bossfights and memorable characters are the core of this game.

Some npcs are outright entertaining in behaviour or the silly things they say, there's no shortage of such characters.
The story dialogue is fairly minimal and characters usually talk about their immediate surroundings.
This is then, where the zone design shines as it does a great job at conveying the story.
There's a lot to explore in every zone and unique enemies make traversing older zones more interesting.
Hallownest seems like a very dark and depressing place but the developers did a great job adding moments that will bring a smile to your face.

On enemy design, it's just so great not to see the same 3 enemies over and over.
That includes both appearance and actions as well as death animations.
Bosses are hard but have mostly clear patterns.
Beating them is tremendously satisfying.

The player character handles a bit clunky in the first few minutes but you get used to the controls pretty fast.
Once you collect several story items, your mobility rises through the roof and traversing zones will become less of a chore.
No kidding; mobility is dialed up to 11 near the mid-game, which more than makes up for your base speed at the start.

Finally, the soundtrack is amazing and definitely supports the atmosphere per zone.
From the relaxing tunes of Dirtmouth to the eerie tones of Deepnest, your ears will be in for a treat.

If I have any con to add, it would be the lack of a sequel.
Hollow Knight definitely deserves a successor, if only to (further) flesh out the universe or its story.

Team Cherry, we expect more great games out of you!

TL;DR:

Great zone design, beautiful soundtrack, memorable NPCs, tricky bosses, a myriad of different enemies and lots of walking.
Get the game. Now. This is a rare case where you do not need to wait for a game to go on sale, it's THAT good.

Just one more zone... OH good loot! ... Another zone!
I've been playing this game since before it was released on Steam and can only say it's been getting better and better.

A definite recommendation for people who like:
- Mountains of loot.
- A (potentially) steep learning curve, depending partly on your race/class/skill choices and partly on the randomized foes which the game throws your way.
- Random events on certain holidays or just... random events really.
- Developer interaction. As stated by some of the other helpfull comments, DarkGod tends to hang out in the global chat from time to time. He also may or may not press his 'SURPRISE' button and spawn a heap of difficult monsters on your path.
- Experimentation. There are many ways in which you can tackle this game and the workshop contains even more classes/races. Some mods might not be on the workshop (yet) but can be found on the TOME4 website.