It's official, primer does help shadow last longer. Up until recently I was a primer virgin. It just wasn't on my list of must haves, eyelid primer at least wasn't. However, having tried out a primer over the past week, I can say my shadow is lasting a tad better. I apply before work and need it to last at least 8 hours. My lids are neither particularly greasy nor dry, but some shadows do have the nasty habit of creasing more so than others.
I've been using ELF primer over the past week with various shadows, all powder, and they have lasted slightly better. I would say though that a fairly generous amount is required and needs to be blended well. Bottom line is that I will be experimenting with more primers in future, I've already got my eye on Urban Decay and Too-Faced versions.
At £1.50 this is a good option for those, who want to try out how a primer performs on them without committing to a pricey item.