I've used it to simulate oil & fuel stains. On the fuselage. Sine the cat attacked my P-40 I made it look like it has been on the ground for some time and really went nuts on the weathering. The oil stains using Tamiya Smoke I think were the best touch. The rest, well..., like I said, I went nuts.
Also. I've been using it lately to make my engines look greasy in some areas. Adds texture. Maybe I'll post a picture of the JRB4s (Beech 18) engines as soon as my web server is up and running again.
In all, I think Tamiya Smoke is an excellent must have acquisition for those interested in weathering. That, pastel chalks for panel fading, and a 0.20mm tip (a.k.a. size 005) pen for panel line darkening.