Have had my eris for about a year. It's a pretty good little phone, but needs to be maintained. A gmail account is a must. With all the apps available, you're tempted to go overboard right away. Go slowly, and look into the apps you're considering, some apps don't play well with the eris!
I would look into a different browser-there are some good ones out there-dolphin hd or opera. A file mananger is also handy, I use astro. dropbox is good for pc work. From there, explore different apps that suit you. If your phone starts to do strange things, go back to the last app you dl'd, dump it, and see how your phone performs.
Every few days clear your cache. The phones' internal storage is not large, and cache can eat it up. Go to menu>settings>applications>manage applications. Let it load click menu>sort by size. Click on each app and hit clear cache. You'll pick up several megs dong this.
Keep checking the forums and they should be able to walk you through any problems. Good luck, and enjoy!