I was first introduced to Shot Bloks six months ago on an 18 mile trail run. I had previously been using GU gels. On my run I made the bone headed mistake of leaving my stash of gels in my car. I had a few in my short pockets so by the time they ran out and I realized that I didn't have any more in my waist belt pocket I was already 8 miles into my run. Fortunately my running partner had a bunch of Shot Bloks and was willing to share. I have never embarked on a long run without Shot Bloks since.
For long runs I typically prime myself right before the run with a GU, banana, a handful of dates and water. Once on the run I eat one Blok every mile and follow it with a swig of water. When it's hot and I'm sweating a lot I alternate one Strawberry (or whatever flavor), followed by one Margarita the next mile. The extra sodium in the Margarita helps maintain your electrolyte balance.
I prefer Shot Bloks over GU because they go down a lot easier for starters. GU requires a lot of water afterward, Bloks don't. Bloks also offer more calories per package. A single GU is 100 calories, a package of Bloks is 200 calories (100/3 Bloks).