Have a look at the HTC phones - available under various names from each of the main mobile guys (the XDA on o2, HTC on Orange and MDA on T Mobile.)
the HTC Touch Diamond, Touch Pro and Touch HD are the latest handsets. all run WinMobile 6.1, but with HTC's own interface running on top.
When it first came out the Touch Diamond was a little sluggish, but the update that came out within a month last summer quickened things up nicely.
the basic differences is:
Touch Diamond: 4Gb internal storeage, smallest of the 3. sVGA screen, no expandable memory slot.
Touch Pro: same as the Diamond, but has a slide out keyboard and a MicroSD card slot.
Touch HD is the largest, about the same size as a iPhone. doesnt have a slide out keyboard, but does have the internal memory and expandable microSD card slot. also has a 3.5mm headphone jack so you dont need special headphones.
all 3 have GPS on board. and google maps installed (or can be installed for free if not) have opera browser
a good place for reviews: http://www.coolsmartphone.com