The section around the Loveland/Ft. Collins exits gets pretty nasty too, but those are also getting to be pretty big bergs anymore.
This ^^^^^
The interchange at Exit 257 is insane and getting worse. Loveland, Windsor and Johnstown all have active presence here, as all have annexed the entire area in recent years.
In particular, Johnstown has been very aggressive in annexation and building along the US 34 corridor east of I-25. And even Greeley is getting in on the action, having annexed and extended their city limits to within a mile of the Larimer/Weld county line.
Weld County is poised to overtake Larimer County in population within the next few decades, as figures I've seen project a population of over 800,000 by 2030. And I-25 will probably still only be 2 lanes north of Longmont.