I work for a company that writes software for Pocket PC devices so use them all day long.
I have also owned every xda from the original.
What you have to understand is its not the hardware that is at fault it is the software - it is the ROM version that you are running on the device that makes all the difference between a nice usefull device and a nightmare.
The O2 software is often a massive overhead that further slows down the device. To have a completely clean O2 software free install just choose corporate during the setup after a hard reset - the password is "0506" (this will work on all XDA's)
The original XDA they never really got it right - it was just about usable by the final ROM release.
The XDA II was terrible at first - bluetooth was useless, etc but by the time they got to the final ROM release for that device they had sorted out most of the issues.
The XDA IIs was fairly stable at first - but after a couple of ROM releaseds it reallyu settled down - and i think this has been the best XDA so far.
The XDA IIi was back to square one as far as I was concerned - Slow and buggy just like the original.
The XDA Exec - is also riddled with bugs which make the operating system slow to respond and the alarms are uselles - they go off when you have them turned off - and dont go off when you need them.
I am praying for vast improvements in the next ROM release.
If you are having trouble with any XDA it is allways worth making sure you have the latest ROM version and upgrading if necessary.
You can get the latest official ROM releases from www.my-xda.com