When I first got my Ally, the Back button was very responsive, even with the original oem full-screen film that comes with the phone still attached. I was very happy about that, because the demo models that I had played with in the Verizon store had very un-responsive back buttons (actually the whole screen seemed kind-of unresponsive).
However, about 10 minutes after I first turned on my phone, it prompted me to install a system update (2.1-update1 I think). After the update, the screen responsiveness went down, and the back button became very hard to activate. Getting the notification bars to drop down (by dragging the top area of the screen) was also very difficult.
Just recently, in preparation for installing the Froyo update, I did a factory reset on the phone. Immediately after the reset, the screen was noticeably more responsive, and the back button / notification areas worked almost effortlessly! Just like it had been originally. I did another factory reset after the Froyo update, and the back button / notification drop-downs still work very easily. And I still have the original oem screen film attached.
I realize that doing a factory reset is a huge pain (having to backup and restore all your info and settings, re-installing apps, losing your text messages, etc), but it totally fixed the screen responsiveness on my phone.