Healers experimenting with shadow will recall these friendly guys. They only hand around for 15 seconds or so, but they’re your best friend for every second of it as they completely refill a mana bar. Shadow priests with entry-level raid gear are welcome to continue to use the SF as a mana source, but once well geared mana issues almost completely disapear.
Since changes with 3.1, shadowfiends now do decent DPS. I personally use mine as a DPS cooldown, and pop it early in the fight. This enables me to use it two or even three times in a fight, for a nice boost to dps. Usually if I was going to have any mana issues, this takes care of it naturally while maintaining maximum dps.
If a gimmick of a fight somehow leaves you significantly show on mana, popping Hymn of Hope before unleashing the fiend will significantly increase the mana return. As a blood elf, I also use this trick to get more mileage out of Arcane Torrent.
I encourage anyone using this fiendish little friend to give him a nickname. Mine is named Rupert!