I've got the Samsung i600, a nice clamshell phone with some PDA functionality (calendar, contacts, tasks, browser, IM, etc.). As a PDA, it's not ideal - it doesn't sync with Outlook's notes and it's built-in task app is pretty weak, but it's a very good phone. For that reason, I've always got it with me - and a weak PDA that's handy is much better (for me) than a great PDA that's too bulky to carry around.
The one killer feature of this smartphone is a built in voice-recorder which is the fastest way to capture ideas on the fly I've ever used - hit two buttons and start talking, much faster than scribbling something with a stylus or hunting for a pen. The downside is you have to review and transcribe them later. Voice recognition and integration with the PDA and phone features would make this unbeatable. But it's still pretty great.