The first level I was generated was huge and took about 8 minutes to fully traverse, compared to 2 for the next. I'm also not a big fan of all the narrow chokepoints between rooms, since the enemies will end up firing there for cheap shots. Teleport helps deal with that, but I liked the level structure from the older demos more (or maybe it was just the forest level).
The boss AI has a tendency to get caught in corridors. For bullet hell types this makes it a peek-and-shoot game that takes forever, for melee types it just means you have to kite them through the whole level.