I've also experienced occasional panics when entering/exiting standby, but only when a JVM instance or Virtualbox were running at the time. Otherwise standby/suspend seem to work fine (though maybe that's just a coincidence).

I'm having a very good experience so far (14.04, 4.0 mainline kernel). Palm detection was broken, but the fix in [1] seems to have solved that as well.

[1] http://stevenkohlmeyer.com/fixing-palm-detect-ubuntu-14-04/

I jumped immediately to Arch, so can't speak to the situation with Ubuntu, sorry. I did walk through the initial out of the box boot/config of Ubuntu but was not impressed with the Dell tweaks and setup process, so dumped it. I'm definitely happier on a 4.0 kernel.