This game came out at the wrong time and as a result has a bad reputation. If we rewind to when this game came out, you either had counter strike for free as a mod for half life, or you paid for a bundle that included half life and counter strike together.
This game came around during that time. For many, it was not worth spending extra money on a game you already had. CS 1.6 was one of the most popular games around and had a huge active community. Condition Zero was not merged into CS 1.6 so this game had its own set of servers, however the gameplay was identical to CS 1.6.
Fast forward 20 years. CS 1.6 is still active but the community is small, with most servers only running dust 2 or a zombie mod. CS-Source is dead. CS:GO has been phased out and replaced with CS2. From this perspective is where I find it most fair to review Condition Zero.
Condition Zero first and foremost has far better player models. They look fantastic compared to CS 1.6. The game includes all the original maps in 1.6 plus updated Condition Zero versions with better textures and improved layouts. Think of it like an HD textures pack for a 20+ year old game.
Unlike CS 1.6, this game includes bots with multiple difficulty levels. You do not have to deal with joining suspicious looking servers. You will find that if you try to play 1.6 online, those still left are very, very good at the game and it is not a fun experience. The bots give you a chance to play 1.6 again on a somewhat fair level.
Performance wise, I have found this game runs great. In 1.6 on modern machines, the player models look like they are lagging/jagged and at times stuttering. Almost as if it is a clay-mation stop motion movie. I play at 2560x1080 resolution and I can say without hesitation this version of the game looks far superior in motion than 1.6
In a nutshell, this game should be best viewed as a re-master of CS 1.6 that came out 20 years too soon.