Today I downshifted from a Treo 650 to a older Samsung SPH-i500, which was the closest competitor to the 7135. The i500 is the other Palm clamshell phone that's still on the market, and it uses Graffiti 1 (I found Graffiti 2 unusable).
The Kyo has a lot of features lacking in the i500: a speakerphone, MP3 and (most importantly) an expansion slot. But the size of the i500 is unbeatable for a Palm smartphone. It's the one Palm phone that fits comfortably in your pocket. On reviewing my actual usage patterns instead of my anticipated ones, size is the one feature overrides the qwerty thumboard, hi res screen, camera, SD expansion slot, and Bluetooth features of my previous uberphone.
The biggest benefit to using a smartphone is that it's a ubiquitous organizer. I usually collect with a notetaker wallet, which is much faster than using a stylus, or even a thumboard; or when I'm walking or driving I can use the i500's voice recorder. The notes get thrown into my in-basket, processed into the Palm Desktop, and synched. It's nice to have my hands free instead of lugging around a paper planner or separate PDA. The only things I have to carry now are my wallet, watch and phone.
Another key advantage is that I can copy and paste support material from the desktop or the internet into note attachments to projects, actions, or events (e.g. driving directions, product specs, confirmation numbers).
Of course, the biggest disadvantage is that something has to give. You're either going to end up with a compromised phone, a compromised PDA, or a brick. But like jjvornov, I use a plain Palm setup with no additional software except i500SMS to enable text messaging. You may not want to be that spartan. But it works great for GTD.
