I love and hate my Eris at the same time. When it works, it's great. It's a bit slow (sometimes a lot slow), but I can look past that considering the price. What I can't look past is the overall buggy nature of the phone. Seriously, it is literally the most unpredictable, buggy device I have ever owned in my life. Is it Android 1.5 or Sense UI? Sometimes it won't call out, sometimes it won't turn the wireless on, sometimes it won't turn it off, sometimes the keyboard won't work, etc. etc. A reboot fixes everything, but give it enough time and it will start freaking out. I have to reboot at least a few times a week.
I am a very forgiving person when it comes to technology. I'm an engineer myself and I realize how difficult it is to make a piece of software that is perfect and runs on many different platforms, but seriously it should at least get the basics right, and sometimes it just doesn't (like when it refuses to make a call). I'm hoping that software updates (and the upcoming upgrade to 2.1) improves things but for now I simply couldn't recommend the Eris to anyone looking for a smartphone that just works without any hassle.
All that being said, a friend of mine just got a Nexus One and [edit] is it a beautiful phone. Blazing fast with a great interface, easily as nice as Sense.