I, too, enabled all Backports in Squeeze by giving Backports an apt-pinning of 500, and it worked well until a week ago.
Xorg was upgraded in Backports. However, this was done in bits and pieces, and also not all architectures were done simultaneously. This resulted in a non-installable xserver-xorg-core, because some inputs weren't handled yet.
On my 64-bits desktop, I just had to wait a few days and everything was resolved. On my 32-bits laptop however, i made the mistake to run "apt-get install -f", which turned things around and left me with a mouse that didn't work (perhaps the synaptics part was disabled? Don't know).
Of course it wasn't THAT hard to fix, but it is annoying. And a good advice NOT to use "apt-get install -f" in these cases.
After two days, all of Xorg was uploaded to Backports and I could upgrade my laptop as well. Good howto was given in the Backports mailing list when everything had been uploaded:
http://x.debian.net/reference/squeeze-backports.htmlI don't think one needs to be extra careful when enabling all backports. I DO think it's not the best thing to FORCE anything. I guess Backports isn't the main priority for developers and package maintainers, and, comparing emariz' network-manager story with this one, there can be some delay in dependency issues.
The reason I use Stable+Backports at the moment is that I like newer apps but suffered from a testing that broke on me just once too many. Other reason is that I am running Gnome2 and I'm not sure about slowly moving to Gnome3. So this is why I keep it this way, running a Sid Xfce install in a Virtualbox.
*edit* on a side note, the kernel is never upgraded automatically!