It's the software, stupid!
At least with respect to the VAIO lines, Sony has tried really hard to build integrated multimedia creation and playback solutions. They've got most of the hardware aspects down well enough, but they have still yet to grasp the importance of good software that integrates well with the hardware. Too often they bundle together multimedia PCs with software from different vendors that is poorly documented, works inconsistently on their own or with each other (for movie editing and DVD authoring, for example). Looking at Sony's offerings, then looking again at Apple's iLife and professional suites, really drives home the point.
Apple can't really help Sony out in this venue unless they license out Mac OS X to Sony. I always thought Sony would be a natural candidate to license OS X for consumer/multimedia purposes, much like IBM is a natural candidate to license OS X for high-end workstation/server applications that Apple can't really target right now.
And I used to be an inveterate Sony brand loyalist myself. My dad did some consulting work for Sony in the 70s and flew back and forth to Tokyo quite a bit. He came up with his own tagline that became a mantra to me: "Sony, no baloney!" Unfortunately that's worn off over the years.
Like all big electronics manufacturers, some lines fare better than others: they still make the best overall consumer-level DV cams, but the quality of their low-to-midrange stereo components really took a nosedive, Panasonic/JVC/Toshiba have caught up on flat-screen TV technology, and Sony got a late start on plasma/LCD technology. Their vaunted design quality has been very inconsistent; some things are elegant and simple to understand, other products are too fussy and confusing. They have "Not-Invented-Here" syndrome (manifested most infamously in the Memory Stick) far worse than Apple ever did. It seems there isn't a strong overriding vision overseeing their product design and targeting. It's a harder task for a huge company with such a diverse product line; Steve Jobs and Jonathan Ive would be hard pressed to duplicate their success in this area at Apple if they were magically transported over to Sony.