I'd say more. They have an industrial appearance, and aren't the kind of amenity that you'd want to walk to from a residential neighborhood, plus tend to attract heavier traffic (literally--lots of U-Hauls) than typical retail developments.
I've run across this issue in many of the jurisdictions in which I've worked. Developers will push the idea that self storage is an innocuous, even benign, use to transition between intensive commercial uses and residential. Logically, self storage does not generate as much activity on an hour-to-hour basis as your typical strip center. However, I am not convinced that the impact on residential property values is less with self storage than with retail or other commercial. Why? Because self storage is UGLY.
I think in terms of transition of uses, self storage should be treated the same as any other use. In other words, if a developer wants to site a self storage use adjacent to residential, they should buffer the hell out of it, just as if it were any other kind of commercial use. The structures should have some degree of decorative detail, i.e., the front ends (the ones that usually do not have overhead doors) should receive some sort of architectural treatment other than a blank steel wall. A nice decorative wall should obscure the view of the units and there should be landscaping everywhere. Efforts to control noise and light pollution are important.
I'm with DecaturHawk....buffer the heck out of it if it's adjacent to residential.
Traffic wise, they are a far less impactful type of development on a daily basis....look at the trip generation rates for a self storage vs. a retail development, and you'll find significantly less traffic on a daily basis at a self-storage facility.
I'm with DH on this one as well. Buffer the hell out of it, make it look better than just block buildings, and hide the security wire/broken glass/spikes/ on a smaller wall inside the place, not on top of the wall.
The character is industrial, with potentially more noise than retail, often more impervious surface, and too frequently, rodent problems. Additionally, lighting is often obtrusive. I favor putting these uses in industrial parks or behind commercial uses like big boxes - not adjacent to residential.