hey, i'm pretty new to the G1 myself, but i think i can answer at least partially your questions:
1) i was a DEVOUT BB user for 2 years. i loved my pearl. but in the week i've had my phone, i'm already a convert. interface is much smoother and faster. touchscreen is a definite plus. full qwerty-style board took some getting used to (read: an hour with the massive amount i text), but you've got more room than the pearl.
2) android works with my 2 yahoo accounts and my hotmail account pretty well, on top of the gmail account i already had.
3) i haven't roamed yet, so i can't help you there...
4) i get mail on the 3G and the edge networks, just like on BB.
5) applications are your friend. almost anything you want the phone to do that it doesn't already, you can get an app to do it, plus a TON more that you wouldn't even think about until you stumble across it. that being said, the only limitation i've come across so far is that the android won't connect to an exchange or OWA work email (which in my opinion is the main drawback of the phone), but it works with everything else.
hope that helps