I read about this alternative, and am a convert. Painting in the Seattle area means high humidity for long periods of the year. And humidity makes spraying problematic.
I got a bottle of acrylic gesso from the art department of my local Fred Meyer, cut it 1/1 with standard acrylic paint, and a little water. It still goes on thick and goopy, but this is good. The gesso shrinks to fit the details of the model.
Final results were indistinguishable from a spray primer, and even a little better since I could control the coverage and get to the underside of the model. Spraying is still much faster, but for a few models, I'll be using gesso.
Pics of the two test models when they are finished. If you're interested in how gesso dries, check
this out.