And before that there was the Ericsson R380. None of them could really be described as working "really well".

If you wanted to you could perhaps argue that the PalmPilot was the first touch only portable device that worked "really well", but that wasn't a phone (and you'll have lots of angry Newton fans telling you that are wrong). Or you could try to make an argument for the Treo, but it wasn't really "touch only".

As someone who has used every device mentioned above (and owned at least half of them), I personally feel comfortable calling the iPhone the first phone with a touch only interface that worked "really well"

I briefly owned an HTC Titan before the iPhone came out. I returned it after a few days and got a flip phone instead. It was just too painful trying to do anything with the finicky stylus on the tiny screen. And yes, it was slow too.