pre-alpha 6 there was a gtk+ regression that caused external displays to flicker when loading applications. This has now been resolved. The bug is detailed at https://bugs.launchpad.net/ubuntu/+source/gtk+2.0/+bug/245383/
By default I find the ALPS touchpad sensitivity to be too low, but this can be resolved by updating the xorg.conf file with:
Option "MinSpeed" "0.75"
(this is added to the synaptics driver section)
For the most part use with a standard D-series D/Dock is also very good... external displays can be configured with xrandr (my D630 has an X3100 graphics chipset; the nvidia tool can be used for nvidia chipsets), USB devices are correctly detected, etc. Audio output via the mini-jack on the dock can be enabled with by checking the "IEC958 Default PCM" switch for the HDA Intel (Alsa mixer)
Currently the main problem I have is with repeated undocking/docking leading to a crash in the ehci_hcd (high-speed USB 2.0) driver... this can be worked around by a) disabling high-speed USB in the BIOS; or b) running "echo "blacklist ehci_hcd" > /etc/modprobe.d/blacklist-usb && update-initramfs -u && reboot" as root. I have logged a bug for this over at http://bugzilla.kernel.org/show_bug.cgi?id=11472. When time permits I will continue my investigation and hopefully come up with a proper fix