I did upgrade also from 5.3.5 and the interface is better. Also the interface seems a little faster. However there are stability issues with version 6 on my laptop (Lenovo T400, P8700 2.53, 6GB RAM). It crashes frequently especially if I try to process more images at once (more 6-8). It seems stable though if I set to use only one core (instead of 2 by default) and since I use Win 7 Prof 64 instead of Vista 64 Business. I guess it wants the entire computers for itself and doesn't like to share when it's set on 2 cores.
I didn't test it yet on my quad desktop with XP Prof 32, I will wait until I get a new monitor and calibrate before I start processing more pictures.
Another thing is it doesn't really behave well between accounts. I would advise to try to install from the account that you are using usually (even if it's not administrator) as it puts the presets only on that account. I did manage to copy them to my user folder but who knows what else it installs only on the administrator account.
Also there is something weird with the imported presets. I tried using the Andy presets. In 5.x.x I wasn't to thrilled about them but in v6 I finally see a difference (in better I mean). Problem is that sometimes the output looks like the defaults DxO preset: very saturated and contrasty, worse than the in-camera jpeg (even for my eyes, and I'm far from being even prosumer); probably it doesn't always use them. It's random, I didn't notice it yet since I changed to Win 7. But then again, I didn't process that many nefs since then either.