We used a new bottle of Games Workshop black primer ($10), Armory black primer (5.95) and Board to Pieces black primer (7.99). Not included were Testors, since we rarely have black in stock and automotive primer, which was discounted because the quality was uneven. Usi did our test, our local multi-time painting contest winner.
GW primer had the best results, with an even spray, even coverage that covered the model smoothly and completely.
Board to Pieces primer had very good results, similar to Games Workshop, with even coverage and smooth, almost glossy coverage. Where it didn't do as well was in multi-model coverage, as the nozzle design isn't as good as what you find with Games Workshop. A single model or two primes fine, but painting a larger array of figures might result in uneven spray as you try to figure out exactly where the paint is spraying.
Armory primer went on evenly, but with a chalky consistency. It covered evenly and completely on the model. Armory does well in a pinch, but it's usually my last choice of primer.
So what do I think? You get what you pay for. I prefer Testors when I can get it, but I use Games Workshop for the most part. For single model painters, Board to Pieces is a giant step up from Armory, with a 20% savings compared to Games Workshop.
I'll try to get some photos.