foundation primer - is it worth using?

Depends on the foundation, weather and the condition of your skin.

Some primers smooth out the skin, so are useful if you have large pores and deep lines.
Some also help with excess sebum, so are good for combination or oily skin.
Some will help the foundation last all day or through out high humidity and heat. So foundations that don't last long or hold up well under certain weather conditions would benefit from them.

I mostly wear BB creams, and thus far most hold up great without a primer. But I do apply primer to my t-zone due to combination skin and some large pores, so that it looks smooth and firm.
 
I really like using a primer. I have one that's really moisturizing and I use it with foundation that has a drier texture (like NYX stick foundation). The primer makes the foundation slide on and blend like a dream.
Another primer I have is a Lumene SPF 25 and it's got a tiny bit of sheen to it. Sometimes I mix it in with my foundation/BB cream just to get the SPF - the primer doesn't work under all of my foundations but mixed in it's really nice.