I've gone from 35-39 fairly quickly recently on my Belf Priestess and I've used Tap/Torrent a LOT.
especially while running SM, Most of my XP has come from grouping there (It's a chance to heal isntead of DPSing) and keeping Mana taps up has been extremely helpful,
At my level, each tap gives you 94~ MP, thats about half of a heal, or 1 full mind flay. So keeping 3 up at any one time (theres plenty of casters in SM) can get me a heal and a half in while i would usually have been OOM. It also lets me interupt their spam spellcastting and healing, which is useful.
The other Belf ability, Consume magic is also rather useful at these levels, I've sometimes been letting PW:F fade from me and not rebuffing it (if i know i wont pull aggro) and just using Fear ward/That touch ability rank 1 (shadow touch? I forget the name, but it costs 14 mana) and then consuming it striaght away to give me 350 MP for free. Anothere nice bonus when I would have been OOM.
So for leveling 1-39, Mana Tap has been great, I can see it getting more and more useless as I get higher level though.
I'm gonna stop writing now.... This has turned into an essay...