I thought Brock Lesnar was great right from when he first stepped into a WWE ring. Not only that, but the dude was impressive in a number of ways... His look was better than Roman Reigns' is now, every time he stepped in the ring during his rookie run he made an impression, he was great in the ring from early on, and above all, he had Paul Heyman who basically alleviated the one position that could have seriously hurt Brock's rise, promo skills.
I remember watching him in a match with the Hardy's (might have been his debut match), where both Matt and Jeff just tee off on Lesnar's head with chairs, and he still stayed on his feet. This was the moment where I knew that Lesnar was going to be special... he was too damn imposing and impressive not to be. I didn't expect him to be champ within 6 months of his debut though. After he won KOTR, I kind of knew from that point that Lesnar was going to beat The Rock, because rumors were flying everywhere about The Rock's imminent departure from the WWE. Still, it shocked me to see it happen, but I wasn't disappointed in the least as I thought Lesnar was actually ready to be champ. Everything that happened from then on didn't necessarily surprise me as by that point, it was pretty obvious that WWE was going with Lesnar as their next poster boy and the mega push was bound to follow.
As for how it affected business? I believe that Lesnar drew pretty well as WWE Champ and his feud with Kurt Angle brought a lot of eyes to Smackdown. As for how it affected TV... Lesnar didn't want to travel a full schedule (Raw and Smackdown) as champ, so the World Heavyweight Championship was eventually brought into the fold. Who knows, if Lesnar had never complained we may never have even got the WHC.