I only ever used it to force my PoM to bounce when it was on me, but that's because I always had shadow to quest as.
As shadow, it really is simply an execute. You can work it into the priorities fairly easily if you want to for the mana gain, but it's a tradeoff. Until the mob hits <25%, it's a small dps loss in exchange for 8% of your mana every 10 seconds.
Also, as shadow, it hits too hard. Seriously. 25k crits are not uncommon in a raid for me in the execute phase. When 4.0 hit, I was in the top dps very very easily, and by a large margin. They had to hotfix the glyph to include a 10 second inner cooldown, since I could just spam SW:D and get 15k dps easily. A very large, instant cast nuke with no cooldown? Oh hell yes.
But all in all, it was too powerful for shadow. I know it sucks for healers, but this was the only way for shadow priests to not become walking death machines past 25%.