While dual clutch gearboxes are definitely a nifty piece of technology, the fact is that they can never achieve the low speed smoothness and fuss free motoring that a traditional torque converter auto can.
If you try an automatic from a x35i BMW, you'd be hard pressed to distinguish whether it's an auto or a DCT. The slushbox in your 320d is geared towards smooth driving and economy and is a completely different ZF unit, so that's why it might feel a bit sluggish compared to the S-tronic.
Torque converter autos have become so good these days that BMW does not see the need to introduce DCT across the whole range. I don't see the need either. The ZF on 135i's is so good that it really doesn't need DCT. In sports mode, it locks up the torque converter past 2nd gear so it becomes a direct link between the engine and wheels, and gets rid of the 'slip' that you speak off. Gear changes give the same 'kick in the ass' feel that I experienced with dual clutch equipped VWs.
For shift times, the steptronic is 200ms consistent for any up gearchange. DSG/DCT are harder to determine, since their shift times are highly variable depending on whether the computer predicted the correct gear change and pre-loaded the gear. The fastest they can achieve is in the 50-80ms range for a pre-loaded gear, but that can go up to anywhere up to 500ms, slower than the steptronic. Of course in a straight line drag race, you'd really be smashing through the gears in a dual clutch system. Don't believe the 8ms figure VW quote, because that is completely ridiculous. That is the time taken for the synchros to move and does not take into account the clutching action.
As for downchanges, the step takes about 600ms due to the need to blip the throttle. The dual clutches take the same amount of time, since their setup offers no advantages due to the fact that they also have to wait for the throttle blip. HOWEVER, if you make an unexpected downchange (such as 6th to 2nd, which in dual clutches is on the same shaft) that can take up to 900ms, slower than a manual driver! On the topic of throttle blips, the steptronic does that extremely well and feels exactly the same as the DCT.
Combine this with the low speed shunting and jerkiness inherent to dual clutch systems that is non-existent in an auto, then BMW is doing it right for sticking with the torque converter. Just look at the issues VW owners have with their DSGs and you'll see why. This is not taking into account the maintenance cost for clutchpacks and the like, plus the low tolerance for increased power via mods. An auto is maintenance free and can usually take anything you throw at it with a simple ATF change.
The new 8speed ZF in the new range of BMWs are even better - shift times are apparently down to 100ms, which is pretty much on par with the best dual clutch systems. To put this into perspective, the DSG on the old Audi TT had a 200ms shift time.
