I'm running stock 2.1 on my original Eris with the Sense weather widget front-and-center on my home screen. I have lots of apps but no task killer. I used to have Handcent but I dumped it to clear space for Swype instead (I also decided that I didn't really like the popup messages, anyway. That's an iPhone thing.)
My phone works well and even seems downright snappy sometimes. We've been together almost six months now and I still can't stop playing with the thing.
My keys to keeping it happy:
-- Avoid crappy apps. Read the comments, look at the screenshots and think twice before installing anything that looks amateurish and poorly programmed.
-- Don't ask it to do too many things at once. Yes, Android can multitask, but be realistic. Our Erises don't have the most modern processors. Doing a couple of things at once is no problem; doing a whole bunch of CPU-taxing things simultaneously -- such as surfing the web, texting and making calls while streaming a long video -- is going to make your phone balk.
-- Check your settings. I'm sure that most of the problems people report are real. But some of the complaints on here sound like the result of incorrect settings that people just aren't aware of.
-- Pay attention to internal memory. I have a LOT of apps. But I get rid of the ones I don't use and I think carefully about how much space an app will take up before I decide to add anything new. In general, I've found that once I have less than 20 MB remaining in internal storage, performance MAY suffer (though not always). Processes slow down or the device may try to reboot itself (or reboot Sense) to free up some memory.
-- Keep it clean. This is related to the memory issue above. At least once a week, I clear out the call history, old emails and old texts. I also clear caches, even though it seems Android eventually gets around to this on its own. It's not just the browser that caches content. The Market does, some apps do, and even some games.
-- I don't reboot often. But I do carry a spare battery so I end up rebooting anyway whenever I have to make a switch.
-- Before you do something, pay attention to the quality of your data connection. I've gotten frustrated with the performance of an application, only to realize that, duh, my data signal was weak or fluctuating. Of course it was unresponsive.
-- "Exercise" the dialer. I have not had the severe lag some people complain about. But I have found that the phone part of the phone is a little hesitant if I haven't used it in a few days. I try to open the phone at least once a day and use it for something, even if it's just checking my home voicemail. It seems much more responsive if it gets used regularly.
Hope that helps someone.