They work well. They are more than acceptable in flashes.
The most important thing though, is that if you charge them and then leave them in the bottom of your camera bag for six months because your "battery rotation system" failed, you will still have a good battery to use. Eneloops aren't the most powerful battery on the market. Their primary advantage is their slow discharge rate when left unused. I think it is something like 80% of their charge remains even after a year. Most rechargeable would be unusable after sitting unused for a year.