Just like Daniel himself, I'm starting to think that what I said in my last post needs a bit of qualification.
I do think it's possible - indeed, very easy - to get a stable linux desktop. As Daniel as more than hinted at, the tradeoff you have is that you don't update *anything* - you set it up so that it works, and don't change it.
For some people - my parents, most light users - that's probably perfect.
For me, that's not what I want. When a new version of Firefox comes out, it will be on my desktop days later. When the window management framework and UI is upgraded (be it Gnome or Aqua), it will be on my desktop days later.
On OS X, this is easy and stable: upgrading firefox doesn't break anything else, and OS updates come out at a regular steady pace - and each time, they're (generally) working and (in most cases) quite a smooth upgrade.
As I mentioned though, I've just installed Ubuntu on a laptop, and I'm writing this post from it. Yes, I'm tweaking things; yes, this makes it a bit unstable - but right now, I don't mind, because I'm bored at work.
So... to sum up: Linux probably can be a "transparent tool", if you use it the right way. The fact that, for many users, isn't, is more due to the mindset of those users than anything inherent in linux yourself.