I don't really have the screen issue - it's not the easiest thing to see, but you can make out enough. Worst comes to worst, you hold the phone in your shadow and you're all set. Auto brightness cranks up the brightness all the way and it works for me.
I haven't experienced any keyboard lag - the only time I really get lag is when it installs apps. You'll want to get a case that protects the camera (well, I did, at least) becuase it does protrude. Normally, it is at an angle, so the doesn't rest on any surfaces, but I still went for the case.
You'll also want an extended battery of some kind, I think. I got the Seidio 1750. It came before my phone, so I never really used the stock battery - who knows, maybe that'll be adequate for you.
The only issue I have is sometimes, the phone will heat up a lot while I'm doing normal tasks - web browsing, youtube, etc. I emailed HTC and they said it can be replaced by the warranty. I don't really feel like sending it out, and my screen protector is already on, and it's really too much of a hassle to justify. It doesn't happen that often, and hasn't seemed to have hurt the phone yet.
I would do it, my friend has an Eris and he's alwasy complaining about the lag. I'm sure with 2.2, the Inc will be even faster, and it's supposed to have better battery life, as well.