Auto killing is dumb because a lot of stuff gets loaded into Android, but for people who use task killers on a manual basis, that's ok.
Just realize that Android has a tendency to preload stuff, so it's a balance of memory. You want at least 40mb+ free minimum. This is an issue for the 256mb devices, but the 512mb devices are usually fine. I try to maintain 60mb+ on my DROID. Anything down to 30s is noticeably slow + redraws.
I think too many people are quick to judge and say "OMG TASK KILLERS DUMB" because Android developers said so. This is as dumb as saying Steve Jobs says we don't need multitasking or whoever at MS said people don't use Copy & Paste. Fact is we all know Android has memory issues when you go down to a certain point. No doubt that Android has a decent memory management system, but if we can already accept that memory use has improved from 1.5 => 1.6 => 2.0 => 2.1 => 2.2, then obviously there's been improvement. Moreover, there's room for MORE improvement. So for people to say "don't touch it" is probably the dumbest thing ever. It's almost ironic considering the amount of Apple hate here and how people bash iOS for not giving you choice and forcing you to live with what the OS gives you. Well memory management is where too many people let Google decide what to do. Maybe that's why the multitasking is iffy too.
Is auto killing the solution? Not really, but some clever use of the "Autokiller" program by adjusting "Min Free" is actually a decent solution. So is using a prog like Autostarts to make sure a bunch of crap doesn't get loaded at startup.
When people say that killing tasks kills your battery, it depends. If you auto kill every 5 minutes, duh. If you kill every 1 hr, it might not be that bad. I'm willing to bet that an auto kill 24 times a day will use probably like 2-3% of your battery. That's like what a replacement launcher like Launcher Pro will use. Big deal. When you kill every minute or everytime your screen flips on or off, that might be an issue because I'm sure that happens MANY times in a day. So it all depends. Personally I used to manual kill every day or two when it got sluggish. Now I just use a MinFree tweak, but even then a reboot every 2-3 days makes my Droid speedy again.