I used to have a Treo 650 for almost 4 years. I tried a Charm at T-mobile the other day and loved it. Compared to the Treo, there are MANY differences. They are similar in form factor, but that is about where it stops.
I have a Droid and love it, but I still miss the form factor of the Treo.
After looking at the Charm, I love it also and would consider it if I were on T-mobile. Sure the screen resolution is no where near the Droid, and is actually less than the Treo. The phone itself felt great to me. It was pretty responsive. I heard that it also comes with an extended battery, so I'm sure the battery life on the Charm will far surpass the Droid because the screen uses most of the battery. I hope this helps. It is hard to compare the two devices, especially because they run on different operating systems.