In P v T(going mech), Phoenix have become a must, if the game progresses too long.Just watch Day9's daily (posted by GRUNT 2 posts above) and you'll see what I mean.
Also, you can't just go Immortals and think you've countered Siege Tanks, it's not that easy. It all depends on the Terrans unit composition, but if he's really Tank heavy, Immortals won't help you at all.
Yeah, because VRs just stomp MarineTankRaven...
No, you'll need ground army with good composition, and later on mix in phoenix to pick up those pesky Siege Tanks.