After having every version of the O2 XDA, and needing to keep all the functionality but wanting a more modern feel and smaller size the HTC Touch Diamond does it all. It even now has GPS built in! The XDA had to have an a bluetooth external GPS box that you never had when you needed it. Just add Tomtom software or the like and it has proper satnav functions.

The real use of calendar to arrange your timetables for hours or even years ahead is the biggest plus for a PDA, with a good sized 2.8" touchscreen to navigate around on too.

The text messaging has a good feature where it lists your texts with your messages to and from the recipient on the same page to save going back to previous texts to remember what you asked them or wanted.

3.2Mp camera with video function too. You can browse photo albums and listen to music directly from the home screen.

2nd gen HTC Touch, the Touch Diamond takes on the iPhone properly, which the 1st gen was not up to. No longer the gimmicky TouchFLO, replaced by a completely overhauled and practical TouchFLO 3D. Web surfing is handled fantastically by Opera Mobile 9.5 instead of Mobile IE; 7.2Mbit/s HSDPA  and Wi-Fi makes the experience that much better.  

Little extra features are great too like resting the phone on its face to trigger silent mode or the phone waking up when the stylus is removed. 

When size counts the Touch Diamond has it at only 102mm x 51mm x 11.5mm (4"x 2"x 0.45"), providing max screen size with minimum phone size absolutely packed with features.