Sorry but shadow priest mana regen is very akward. In most cases, all your mana regeneration spells will be limiting your DPS unless you are using it for a dps purpose which will then limit your mana regeneration.
On fights where you can squeeze your execute ability in on adds during the whole fight (magmaw heroic), or fights with a lot of raid burst damage (Chimaeron) you are usually staying high on mana throughout the fight. On fights like Maloriak heroic you almost want to be full on mana when you enter the add phase (dark vial), and even during the add phase you want to spend an archangel/shadowfiend cooldown before you enter the execute phase where you will be able to pump up your mana bar again.
I know mana becomes less of an issue with gear scaling which I have experienced myself, but if you want to maximize your damage with the abilities your class is designed around you'll often find you very low on mana. Some times you'll end up using sw:d on cooldown even though it only benefits your mana bar. Other times you'll find yourself using shadowfiend even though your internal cooldowns will be ready again in less than 15 seconds, and at last you are also going to hate yourself for having to use archangel before you refresh your dots, or 25 seconds before entering execute range. Second worst case scenario would still be having to cast dispersion, and clearly worst case scenario is having to cast hymn of hope to get enough mana to cast sw:d.