I bought a Dell e7240 couple of weeks ago and put Linux Mint 16 on it (since Mint is based on Ubuntu, I wouldn't expect the experience to be vastly different for straight-up Ubuntu). Everything except the hardware Wifi on/off switch works out of the box (the Wifi can be comfortably turned on/off via the Cinnamon network applet). Considering that this is new hardware (Haswell i7, 8GB DDR3 RAM and dual 256 GB SSD), I absolutely wouldn't have any issues recommending Dell for Linux to anyone.

Finally, the above system can also be ordered with 12.04 LTS which means Dell is officially supporting Linux. I don't think it can get much better than that.