The Macbook and the Compaq are not a good comparison, the Macbook is much better.
The current Macbook has a faster processor (2.26GHz) and a bigger disk (250Gb) than the one list below and is the same price (£799).
The Macbook processor is also dual core while the Compaq's is a single. While not quite twice the processing power that's still a very good performance boost.
The Mac processor also has more memory within it which helps it to function faster, I think it's about 6 times the amount.
You also get a slightly better graphics card in the Mac.
The Compaq also comes with the home version of Windows 7 that is lacking several features that come with the professional version that could come in handy. For example drive encryption (standard with OS X) and XP Mode (could be very useful for older Windows apps).
That also touches on Windows' confused versioning, there are many and the differences are complex. Mac OS X is Mac OS X, one box, one price, all the features.
So basically it is a clearly better machine than the Compaq. Is it double the price better? A fair question.
Personally being able to use an OS that is virus free is a huge benefit to me that is hard to put a price on.
My final point would be the build quality. Cheap PCs tend to feel, well, cheap basically. Macs are well built solid laptops and they now all come with a very nice and clear looking screen.
Go into a Mac shop and PC World and just compare the differences.
Again is that worth the extra?
Is an expensive BMW worth more than a beat up old Ford?
Both get you from A to B.
It's a personal choice.