I'm a USC fan, but the more I learn about Leinart personally (he's a ####) the less I like him. That said, I'm ambivalent about him overall and therefore pretty neutral.
I think he's going to be "fine", and by "fine" I mean worthy of a starting QB job somewhere in the NFL. Will he be a Pro Bowler or a fantasy stud? Who knows?
Everyone needs to relax. As I said in the new coaches thread, the negative effects upon a team of a transition to a new coach and scheme always tends to get underestimated. Last year, when Saunders went to the 'Skins, everyone started talking about huge passing yards and rushing, etc., failing to remember that it historically has taken a year - you read that right, one year - to fully install that offense and have the players absorb it and "get it". Even if that's an extreme example (and it might be), the fact is that we're at week 1.
If I'm a Cards fan I'm pumped up about what I saw last night. That line looked like a pretty good run blocking line. Edge looked like he did with the Colts. In other words, the thing that everyone figured that Whisenhunt and Grimm would make the priority - o-line play and the running game - were in fact the priority and looked vastly improved.
Pass protection and routes, sight adjustments, blitz pickups, etc. are more complex and take time. The good news is that Leinart is universally respected for his football smarts and he's got arguably the best tandem of WR's in the league to work with in getting that right. He's also in a division with relatively weak defenses. Declaring him a bust or the Cards passing game as a failure this early is silly.
IMHO, if you have him you hold him; play better options if you got them. Leinart is probably a decent buy low candidate, especially as a QB2.