Before reading further it's important to emphasize that itch.io bills itself as a provider of "indie game" content, content from independent developers, as opposed to professional game developers. There are a few commercial products that can be found there that have a presence on platforms like Steam, but most of the content is by smaller teams and single amateur developers.
With that in mind, the site itself reflects this demographic. It has a "pay what you want" model for anything you can acquire, even free items (payment for free items is to support the developer if you desire), and if you are intending to stick to free items only, this service has quite a bit of accessible content of that nature for the gamer on the cheap.
itch.io does have a cross-platform game launcher for Windows, Linux, and macOS, and this review is generally written with it in mind, as it makes for a far easier means of browsing itch.io as opposed to a regular web browser. It's not essential like Steam's client, but it's still highly recommended due to its ease of use, especially for keeping track of the content you follow and have downloaded.
Like Steam, the service provides not only games, it also provides game-related tools and assets. In this, itch.io is in some ways superior to Steam. itch.io caters to the game developer crowd extensively and provides a ton of content, most for free, for the budding developer to use as a base for making their own game in any game engine of their choice.
As to the actual games, this is where the mixed bag effect comes into play. While there are some great indie games to be found, both free and paid, the site has the feeling of a giant warehouse-sized flea market. One has to sift through a lot of miscellanies to find the hidden gems among the marked down mass of trinkets. There is a tag-based sorting system for filtering through some of the clutter, but the gatekeeping for content does not preclude (very well, at least) in-progress projects or unfinished projects. This leads to a lot of noise versus actual signal to filter through for actual finished/playable games. There is also a lot of adult and LGBTQ content, and while the former is supposed to be filtered depending on preference, I found the filters aren't nearly as effective as they are on other services.
For those wishing to upload content, the site does have a low bar to entry so long as you have legitimate content to upload and their rules and policies are obeyed. The game client is again preferred here since it makes it easier to keep track of your uploaded content. The actual policies are fairly reasonable in large part and compared to Steam any fees charged for use of paid services (such as the cut derived from your sales) are usually no higher than is determined by the developer, so long as the product is sold for any amount above zero USD.
FINAL VERDICT: RECOMMENDED
itch.io is a pretty good service for the game developer and those who are looking to see what the indie game dev market is producing nigh exclusively. Sure, there are a few issues as noted concerning sifting through a lot of silt to find the hidden gems, but if you don't mind doing a fair bit of browsing, there are a lot of game assets, indie titles, and even books one can acquire here. As an added bonus, unlike Steam, your wallet can be empty and you can still walk away with some good content. For professional content, stick to other sources, but itch.io is certainly a great resource for amateur game developers and indie game fans.