I want to first say that I am not nearly as knowledgeable as the other posters. But want to add my 2 cents. The first few days of having my eris the battery life was horrible. Did good to make it 9 or 10 hours, and I really wasn't doing much with it. Then I found this forum. I went through the battery life thread and the only adjustments I made to my phone were reducing the screen brightness, turning off phone location and gps unless I need it or I'm travelling, and installing spare parts(to watch battery level), and switching from the stock browser to dolphin. And then I went through 4 days of draining/charging. At the end of the second week, my battery was doing great---it was consistently making it through my waking hours(16-17 hours), being around 15-20% when I plugged it in at night. Then I decided I would go back to the stock browser(dolphin is a bit more complicated, at least to me, so just thought I would go back to the stock one). I did that at the end of the day on Saturday, the 22nd. On Sunday, my battery lasted 10 hours, and I did not use it hardly at all, but didn't really need it, so didn't worry, thought maybe it was just the signal or something(my signal is horrible, live in the country). The next day, I did use it some and it lasted only about 8 hours, I was completely bummed. I knew the only change I made was the browser, so I reinstalled dolphin, made it the default browser, and wow, my battery is great. I was thinking of getting the seidio 1750 battery, but it is doing so well I'm not sure now.
OH, and my signal is horrible most of the time. Most of the time only the tiny bar is lit up, and it goes between 1x and 3g all the time. It is better when I go to town, but I am a stay at home mom/ farm wife, so am in the country the majority(if not all) of my day. I was told at my verizon store that htc's just don't receive the signal as well as other phones---motorola's receive signals better(the moto droids are getting 3g all the time in our rural area). So I think that must just be an htc thing. (and no, they weren't trying to get me to change to the moto droid--they don't like it, none of the workers there have it, they have the eris or bb curve--three salespeople there, all local ladies that I have known for years)
Ok, I'll quit rambling...........don't give up on your eris, it is such a great phone, just be patient with it!