I had the same problem the first week when the new talent patch hit, I do recomend you to use your shadowfiend when you need it, as well as dispersion. Get the glyphs for both shadowfiend and dispersion while you're at it, if you're in the need of mana, the 45 seconds on your dispersion is really quite alot, and when fighting a boss that does AoE the glyphs for your leeching friends death is prettty handy also
This is just my whole experience as a level 80, I haven't checked if it will remain the same at 80+