Critical hits are still possible in conjunction with auto-crit skills (attacking out of stealth, Pinpoint Strike, Death Hex). All attack rolls which would normally be critical hits are converted into normal hits. All attack rolls which would normally be normal hits are converted into critical hits.
One of the three possible animations does not perform attack rolls. As such, 1/3 of attacks will never connect at all. Even with these misses, dual striking still generates 33% more hits than attacking normally. An unofficial fix for PC can be found here.
The off-hand weapon is not taken into account when determining attack speed, only the main-hand. For example, a dagger (main)/longsword (off) setup will attack 40% faster than longsword (main)/dagger (off).