I've run a few now, and I'd say while obviously they were more like D&D than a Call of Cthulhu game, they were all good, if not great, changes of pace. Adding some form of Madness (as per DMG) can easily be done, but isn't essential.
And Madness Follows, Dungeon #134, was a great 3.5 urban adventure which I used as a side-trek for mid-level PC's.
Last Breaths of Ashenport, Dungeon #152/156?, was a great 4.0 (and 3.5?) adventure set in a place that's definitely inspired by Innsmouth, so could easily be the Purple Rocks. From memory, I ran it under 4.0 rules, again as a bit of a side-trek. I thought it might have been released under 3.5 and 4.0 rules, but I might be getting that confused with another adventure released around the time 4.0 was brand new and Dungeon was newly digital and in the process of converting over from 3.5 to 4.0 adventures...
And Madness Follows, Dungeon #134, was a great 3.5 urban adventure which I used as a side-trek for mid-level PC's.
Last Breaths of Ashenport, Dungeon #152/156?, was a great 4.0 (and 3.5?) adventure set in a place that's definitely inspired by Innsmouth, so could easily be the Purple Rocks. From memory, I ran it under 4.0 rules, again as a bit of a side-trek. I thought it might have been released under 3.5 and 4.0 rules, but I might be getting that confused with another adventure released around the time 4.0 was brand new and Dungeon was newly digital and in the process of converting over from 3.5 to 4.0 adventures...