Yeah, I can't see siphon as being a terrible power, unless you're expecting it to do everything on its own. For all that people love cannons, one on its own is only going to do so much. Siphon is a nice addition to your own power levels (and we all scrabble for a bit more power in x system all the time) and it weakens whatever you're fighting. Works great for me, and it's even nastier coupled with Polarons. I've been amazed at how fast some ships will have their shields go down with Siphon and my other weapons going.
That's actually why I'm wanting to dump Scramble Sensors. Things are fine with Siphon and Gravity Well going, but I need something else when those are on cooldown. I may try charged particle burst to see what I think, since I already know Tachyon Beam well.
Oh, and on that note, people are hating on Tachyon Beam more than I would. No way am I going to argue for it on a Cruiser, but it's another nice ST attack, in my book, one that doesn't drain weapon power. If you're looking for that and maneuverable, it's decent, though I would argue for a little upping to its drain.