What Windows Mobile 5.0 AKU 2.0 has and Windows Mobile 6 (the cooked ones, having tried them all) doesn't have, according to my experiences:
1. Videocalling - at least not a proper one
2. Bluetooth Dial Up Networking (the DUN profile)
3. Wireless Modem (yet some WM6 ROM's have it, although without shortcut in the start menu)
4. Cyberon Voice Dial by default (some WM6 ROM's have it by default, others will need a 500kB CAB to activate it)
The reason why I went back to "good old" WM5 is mainly the reliability. Ok, the OS may be a bit slower here and there, but it has all the functionality (e.g. videocalls and Bluetooth DUN), and NEVER hangs. It just always works and that's what I desperately need right now.
The things Windows Mobile 6 has for me that 5.0 hasn't which I miss the most:
1. High resolution browsing in the normal Internet Explorer (I don't want to use alternatives)
2. HTML email support
3. The impressive speed of the OS
Applications like WM6's Windows Live and the superb Remote Desktop Client are also available for WM5, so I don't need to miss those, otherwise I sure would, and then I might have sticked with WM6.
The new sounds and GUI are great, but I don't really miss them, since that's not a true functional thing.
Having WM5 up and running again, without freezes and memory leaking and with full HTC Universal functionality, is enough for me to be happy with it and to stick with it. If there ever will be a ROM that actually ADDS Windows Mobile 6 on top of all of this (without a single registry tweak or CAB fix needed), then it's the time for me to upgrade again.
If such a ROM will ever see the daylight, since it won't be officially available for the Universal, I don't know. I think the HTC X7500 (Athena) will be there sooner (April 2007 over here), and that one will have a official WM6 upgrade for sure. That'll be my next one, so until then: WM5 for my Universal!
So far for my opinion.