IdleOn is an idle game heavily inspired by Runescape and Melvor Idle with 15 (soon 18) different skills that are all connected in a convuluted web resulting in a very engaging gameplay loop where you manage up to 10 characters of different classes. The main drawback is that there are many baffling design decisions, a barage of bugs and several ways to permanently mess up your account. It seems like the solo dev never returns to fix issues and only focus on adding new stuff.
In contrast to games like Melvor Idle or Cookie Clicker this "Idle" game doesn't really cap how much you can play in a day. There is diminishing returns, but given the huge amount of different tasks and systems you will never run out of things to actively do before the end game (which I have not and will not reach). If this is a positive or negative is up to you.
As usual for this genre you will probably want to use external resources. The wiki is servicable, but often lack numbers, doesn't fully explain the math, can be hard to navigate and doesn't always explain the fundamentals of skills making it difficult to read ahead. You will probably want to join the discord since talking to other players in game is god awful. If you don't have a US keyboard you may not even be able to type properly (yes seriously).
As a free to play game there are micro transactions. It started out with gems that can be used to purchase a lot of quite powerful upgrades, but given that you can get a quite large amount of gems daily by playing the game it isn't particularly objectionable. Then packs (bought with real life money) started including benefits like +10% damge, 20% extra exp or 1.2x drop rate for $10 to $30. These are benefits you can never get free to play. In summer 2023 pets were introduced with the best one allowing you to have 8 poweful bonuses activated at the same time while you usually have to choose just one. This is game changing. To get this F2P you will probably spend 2-3 years and if you pay on average ~$300 with a mercy at about ~$500. This has made a lot of people quit and messed up balancing. It's likely that new pets with similar bullshit will be introduced in the future. Auto-loot is a feature that costs $5 and is almost necessary to play the game since having a window open with the game running for hours is heavily encouraged after a certain point. I personally bought this, a starter pack for $5 and the smallest amount of pet gems for either $5 or $10 (I don't remember).
Other bullshit includes the possibility to permanently mess up your class rooster since you can't change classes (with one minor exception). This is especially likely since there is a secret class which you likely will miss if you don't read the wiki or guides. The buffs from this class are so powerful that you are essentially screwed until another character slot is added which may take many months. There is a tracker for which items you've picked up. If it's given to you directly or you craft it you will need to drop it on the ground and pick it up again. If you don't know about this you may lock yourself out of getting quest items to count and need to recraft a ton of stuff. To top it all of it's hidden until world 5.
All in all I find it difficult to recommend IdleOn. It's very good at pressing all the Idle otpimization buttons, but there is simply too much crap and I can't say the future looks particularly good with the dev seemingly getting greedier and greedier and there's no indication that they intend on going back and fixing issues instead of adding new stuff to an already borderline too complicated game. Go play Melvor Idle instead if this seems like something you would like.
2.5