Here are my reviews of two of the most commonly used primers, certainly by beginners who will probably pick them up when recommended by GW staff:
I've read some real hatred for this primer around a few sites. I'm not sure why, as I've found it to be reasonably good when undercoating ogres and orcs. Some people say that they've had issues with the spray ending up grainy in the finish but I can only assume on the basis of my experience that this might be due to atmospherics or perhaps bad technique. I would, however, give several of the primers from the Army Painter range a try before getting this again.
This has been a good primer all round - used on a good batch of my Marienburg army, plus Brother Paulo's Brets. As with the Chaos Black spray, I've seen some criticism about graining, but this only happened once for me, and it was entirely my own fault because I nipped out to spray just after a rain shower: lesson learned.