Out of curiosity I downloaded and installed two screen apps last night, Launcher Pro and Helix.
Both are just as fast as Home and Sense on my phone.
I really have use for Launcher Pro's landscape mode on the home screen, as I use my device in that mode as often as I can, just a preference.
I'm not sure yet, but I just may keep Launcher Pro. Helix is being deleted, though, not because I hate it or had problems with it, but just because I'll not need it.
I've never had lag with my Eris and I feel badly for those who do and are trying to make it run as it should. If I had that issue or any other persistent one I think I'd be at the Verizon store getting replacements and testing them right there in the store and not walking out the door with it until it was the one that operated as is should; I'd be willing to spend a lot of time getting it right.
As it is, the one they handed me in January has always been great; I really thought they all were that way, that it was just good HTC quality.
It turns out I was wrong about that, unfortunately; it appears the quality control at the factories where it's made, assembled and inspected is lacking, otherwise they'd be consistently just like mine is.