Benchmark tests
I'm confused about many of the above comments about benchmark tests. Having recently done some research to find a good thin and light with all day battery life, it seems (in this category anyway) the mac is pretty much equivalent to a windows one, in terms of performance.
Laptop mag, for example ( http://www.laptopmag.com/review/laptops/apple-macbook-2010.aspx ) if you do a comparison with similarly priced (and yes i know this is a US site, and UK prices don't usually compare directly) Asus / Lenovo / HP, the benchmarks almost all reflect the price bracket. The Macbook slightly outperforms the Asus U30Jc, and is slightly more expensive. The 320m graphics outperforms many discrete graphics cards, and you don't have to worry about switching technology. Several reviews say the U30Jc's keyboard and trackpad are not as good as others.
The PCMark Vantage test shows the mac's hardware disadvantage, but only because that test has to run Windows 7 in bootcamp. The conclusion being that OSX can perform as well or better than Windows 7 on inferior hardware. So if you feel you are being ripped off by older processors and less RAM, then get a PC. But if you want a portable laptop with good performance and long battery life then it's just a case of which OS and/ or brand you prefer, and how much do you want to spend (bearing in mind no thin & lights are cheap).
I went for the macbook in the end (having been a desktop PC user until now), and am not regretting it so far (2 months later). Keyboard and trackpad are worth the price of admission alone - BIG considerations for a laptop, having tried a few others with wretched trackpads. You get about 7/8 hours of normal use from the battery, without having to resort to a ULV processor.