Same thing with Spelunky. Its impressive how much emergent behavior you can get by putting some mario-style platforming on top of the procedural generation.
Speaking of procedurally generated platformers, there's Cloudberry Kingdom.
I can see how you could experience remix boredom in Cloudberry Kingdom. However, every so often it produces novel gameplay that actually feels new. And it's so good at tuning the difficulty to gradually escalate that it manages to achieve flow quite effectively.
Cloudberry Kingdom actually did feel like it created new stages every single time. It did a great job of mixing up gameplay just by tossing in little modifiers every few stages.
Unfortunately, it's fairly rare for people to beat that game. The difficulty becomes basically absurd right up to level 320. At that point you essentially have to turn on all the powerups because there's no way you can get through without them.
I played through that game so many times that I got to New Game + + and all enemies were minibosses. Remix boredom set in at that point though, since with the majority of enemies gone things got a little samey.
I can see how you could experience remix boredom in Cloudberry Kingdom. However, every so often it produces novel gameplay that actually feels new. And it's so good at tuning the difficulty to gradually escalate that it manages to achieve flow quite effectively.