I think mupak is pretty unique in that it really strives to create fleshed out locations. From the seamless skyboxes, to intricate sector work and tiny little details, these are (mostly) believable locations which just so happen to also be racetracks. That's quite a feat considering the engine, and limited use of custom textures.
It's also one of the few map packs that puts a good chunk of effort into encore, with music changes being fairly common iirc, and most maps changing in some way.
The layouts are mostly fun and varied, although the level of detailing can sometimes result in a few too many things to get snagged on. The only maps that really stick out to me are Neon Sands and Cascade Glade, which feel a bit less engaging due to an abundance of long straightaways.