I used them quite a bit at the pool I used to lifeguard at, and I was never really that impressed with them. They worked, but they didn't stay cold for very long, and we'd end up using a few of them to treat a single injury. Who knows, maybe we just had the cheap ones?

Personally, I don't think they are worth their space or weight in a back pack. I would rather carry an extra water bottle. You could use a cold pack to cool off, but I don't think it would last long enough to be worth carrying. If it's a really hot day out, then as JRR mentioned, they may have some value as a drink cooler. However, in a car or home kit (where space and weight aren't as big of an issue), they are better than nothing and probably worth having around.