I don't have the expertise to judge her voice but its richness when she really belts it is wonderful to my ear, and she also has that richness in quieter songs like "Cry Me a River" and "Killing Me Softly" -- old performances of hers that have been found and posted on the net. None of it is my kind of music but I really enjoy the sound of her voice in them.
People who say, rightly or wrongly, that there are other voices better than hers are missing the point. She's been a contestant on an amateur talent show, someone who always wanted a career and is good enough to have it. She doesn't have to be the best who ever lived to qualify for that.
I appreciate what Norman Lebrecht had to say about her voice recently, in his colorful prose:
...a voice as full as Loch Lomond in flood.
...a voice that seemed to come from nowhere, delivering show tunes like a pro.
...Boyle, with a voice that is reputedly untrained, works with comfort within a contralto range, taking the top in her stride and delivering arias with little dynamic variation. The vibrato is well controlled and the pitch is, unlike the still-touring [Paul]Potts, pin-point.
...She could pass any professional audition without difficulty, provided it was a blind audition and the judges weren’t influenced by her middle-aged, dull looks.His assessment of her voice is to be trusted I think, because he's a music critic by profession.
But then he sums up her talent as "small" which leaves me wondering.
Millions from Kenya to Korea will be rooting for Boyle to win on Saturday night, not because they admire her small and very specific talent but because they live in hope that her dream could somehow be theirs. Boyle is the first heroine of the Yes We Can universe.
He goes on about the fantasy of success her story seems to have generated across the world and I suppose there's a lot to that, and he's not the first to say it. But it's still her voice itself that I enjoy the most, that to my mind not-so-small talent that is "as full as Loch Lomond in flood."