At the risk of stating the bleeding obvious, I'd decide on which features that you're actually going to use and how much money you've got to spend, and work around that. Something that has built-in "extra" features (e.g. camera and phone) may not be particularly good at them - I've briefly used an XDA Exec (HTC Universal) and found that the be the case there.
I'd also try and "have a go" with the various operating systems supported (e.g. Windows Mobile, Palm OS etc.) and see what you think would be easiest to live with.
If you think that you'd want something running Windows Mobile 5, then I'd check the forums on
modaco.com. I'm sure that someone can recommend a similar Palm Treo site. Many of the Windows Mobile 5 devices (such as the O2 XDA Exec) are branded HTC devices, and are also available from other manufacturers.
Once you've got a shortlist I'd try and find out specific info from device users - e.g. how much battery life you really get rather than how much is claimed by the manufacturer.
Personally I use a Blackberry, and if email had been at the top of your feature list I'd have wholeheartedly recommended it, but as it wasn't I'd have to say "write down what features you want and work around that".