I've had an old Medion pocket PC thing running pocket PC 2003.
an XDAmini II (HTC Wizard) that ran windows mobile 5
and I've got another XDA that runs windows mobile 6.5 now.
I think that WM5 was better than WM6, the change was too noticeable. lot of things appeared to change, not necessarily for the better.
the WM6.5 phone that I have now has a faster processor than my old phone and more memory, but it's really sluggish in comparison,
I've got an iphone too and for everyday use I like that a lot more. Partly due to the way the interface works, the screens also bigger, and clearer, it's a capacitive screen whilst my WM device is a resistive screen that's just not as responsive... on the other hand, if you put a capacitive screen on a WM device you'd need a special stylus to be able to write on it or possibly not be able to use a stylus at all and that would take away from it...
pocket PCs are good for working though, if I'm on a client site on in a datacentre doing an audit it's great that I can take notes, draw diagrams, and just drag the word document straight off the device when I get back to the office.
in general, I really think that the difference between Windows Mobile and Iphones OS is very much like the difference between a PC and a MAC, one is more like a dedicated business device, you use it to get work done, there isn't a whole lot of joy or fun in it, but it's firm, it's functional and it mostly works, assuming you want to follow the pretty rigid rules of using the device...
the other device is less of a business tool and more of a pleasure to use, it's sleek, it looks good and it's easy to use, the downside is that it's really clearly not a business tool.