Mr. Surfacer is a very good primer and I've been using it since about two months ago. It dries fast, smoothly and durably (I'm talking about the Mr. S 1000, which is light gray in color). It's lacquer-based, though, so you may have issues in using it. Anyway, it's a little thick for airbrushing so I thin it a bit with household lacquer thinner. You can also use Mr. Color Acrylic Lacquer Thinner for thinning. All in all, this is good stuff.
Haven't used the Base White yet but I've read that this is good for undercoating dark-colored plastic, as gdarrington said. It's also great as an undercoat when you need to apply a light or translucent color as a topcoat, such as white and yellow, which are a pain to put on specially when the underlying coats are darker.
Cheers,
onyan