Yeah I can confirm it's rubbish. My step daughter used to play it and loved it so I gave it a try. It's literally like a map mod in L4D where you just keep fending off zombies, building barriers etc. Like, survival. Unfortunately if you have ever played a proper campaign in L4D then it's just awful.
I use to enjoy the Zombie Mode in World At War and iirc Black Op's because you use to be able to turn stuff on like Paintball Ammo and it just made, it more silly and fun.
Since COD games decided to get rid of the "Console" where you could type stuff in, the Zombies aspect for me has gone downhill.
Zombie Mode is well worth 15 bucks! Just not at this point, because Treyarch seem to not give a flying f**k about the Zombies community with BO4. The mode itself had some serious depth for the last entries with hidden easter eggs and an entire wicked storyline (multiple actually). It's definitely NOT just fending off waves as there is SO much more to do and explore.
I haven't touched BO4's Zombies mode yet as well, but played it in BO2 and 3 extensively.