I've seen a few of them on the servers. Some of them look fantastic, some of them are very hard to tell apart. As a disclaimer, I cannot stand WorldPainter maps, and I really value originality in the theme. Anyways, quick impressions of the ones that I've seen in descending order.
Icarus: This one isn't on the test servers, but I was able to download it and take a look around. Completely original, looks great, and you can tell it was designed with gameplay in mind. My only complaint was that some of the larger rooms feel a bit empty, but time constraints will do that. The asteroid material made me chuckle as well - great choice there.
Colossal Caverns: I haven't seen it on the servers thanks to various errors, but once again I downloaded and took a look about. If you're going to go with the forest/landscape theme, this is how you do it. Absolutely massive and a great sense of scale. The floating lapis blocks at spawn are a great touch.
Quartz Laboratory: I haven't been able to spend nearly as much time with this one as I'd like, but I have to give it points for originality. Players certainly seem to enjoy it.
Rusty Survival Games: Amazing detail. Looks less than great with the regular texture pack. Most of the impressive structures do not appear to have an interior. Definitely designed to look jaw-dropping in a trailer at some expense to gameplay.
Excavation Zero: Spectated a round with this one. Looked great in the screenshots and the structures are amazing. But most of this map appears to be a very similar-looking forest. With more time this one may grow on me.
OTHERS: I haven't spent enough time with the submissions to be an effective judge, but my number one complaint is that most of these maps look a bit similar. Forest+caves+structures. One of the maps appears to have a major importation error where half the map has been replaced by default chunks, destroying the dome in the process. The working half looked decent, but when it takes me a few seconds to realize that there is a clear difference between the randomly generated chunks and the PMC-finalist map, something is wrong.
Totem Orbum: Worldpainter. Worldpainter as far as the eye can see. Complete with snow-capped mountains even. Not sure why this was added. Just look at the overview screenshot. One size brush used for generating straight ridges with an equal distribution of snow. The obsidian ridges look particularly unnatural.
Hell in Heaven: This is one of the real head-scratchers. I can appreciate the effort behind the build, but you're given a list of 500 maps and you pick this one for testing? The hand-built sections like the floating island look completely out of place next to the worldpainted landscape - the overall effect is very jarring. The area around the spawn (particularly the perfectly-rounded volcano) did not make a great first impression.
SUMMARY: All of the maps that I've reviewed (aside from the broken one) are impressive. My one complaint is that some of them are a bit harder to differentiate from others, which of course would be solved by spending the appropriate amount of time with them. The final two are in the top 40% or so of the submissions, but likely should not have been selected out of the 500 maps.
