I own both an Eris and a Droid X. My X is my primary phone and I usually only bust-out the Eris when testing things related to root or playing with Android app development. I have no plans to abandon my Eris. I love that little guy and it is still an awesome phone (especially when rooted).
Yes, the X is a little bit "slabby", especially when compared to the sleek lines of the Eris. But I'm very comfortable with its lines and feel in the hand (I didn't care for the Incredible and I didn't want a slide-out keyboard). The X is also much faster in all respects to the Eris and the much larger screen is incomparable. Viewing and typing is awesome on the X (although I usually do change to landscape mode for typing since that seems to work better for me).
I actually have not rooted my X (yet) or installed the leaked version of Froyo thats floating-around out "there" (I know better than to install "leaks"
). The base 2.1 X can be slightly overclocked (but the kernel cannot be replaced). The devs are beginning to be able to build custom ROMs. Things are beginning to open-up in this regard.
I jumped on the X since I don't think we'll see too many other new phones with 4.3 inch screens...I think we'll see the 4-inch and below in the new, sleeker forms that we're starting to see now with the new phones that are being released.
Cheers!