Personally, I think her voice gets stronger with every album. We saw her true voice on Stripped, then even もっと見る greatness on Back to Basics which btw in my opinion her voice was even もっと見る powerful in. And I've heard the 18 tracks on Bionic, and I can honestly say I haven't noticed her voice declining..it just keeps getting better.
Bionic is epic <3! And 'You ロスト Me' is one hell of a ballad!