I just started using the Thunderbird (well since January) and I must say I love this disc. I know there are a lot of similar discs out there. But the two I have are just perfect.
I have a Champ that is a little lighter in weight (168) and has been worked in. Its a great thrower, great control, still stable-overstable, but not a firebird of course.
I like when the Thunderbird is worked in, as I am a Valkyrie thrower, I am happy to see some turn in the disc at high speed, but I can absolutely trust it that it's not flipping off to Narnia.
I have a 175g Glow Thunderbird that is a slight bit more OS than the other, but not a huge difference...its the same disc of course.
In situations where I may hyzerflip the Valk or Roadrunner I can now confidently throw the Thunderbird flat and have a similar result. The Thunderbird has a similar fade as a Valk, but holds the straight line longer.
Roadrunner, Valk, Thunderbird, Firebird.....I love that setup...and its all I need for drivers in many courses that are not too long or are presenting a good amount of wooded holes, the control of these (speed 9) drivers are perfect.
My rating on the Thunderbird would be 9/5/-0.5/2.25 haha but for simplicity lets just call it accuracy and control driver #1