Yes, we run the slant alot. They, however, require alot of timing, a quick release and very good accuracy. Also, it gets somewhat harder if you're running for your life 4/10 plays behind a shoddy O-line.
Cassel, from what I've seen in highlight vids and Patriots stream, usually stand behind his O-line for ages, eventually finds Welker who runs some kind of drag, in, out, or whatever and Randy Moss pulls double coverage away every time. I was amazed by his still somewhat high average, but I think that has something to do with coverage switches between Moss and Welker. I could get Moss the ball in single coverage and I haven't ever touched a football.
Another factor is this... Bellichick is an excellent coach. I don't like to say it, but he might be the best coach there is, looking at the passing game.
Like I said, I don't think he'll be a complete bust, but I think he'll be somewhere in between Mark Brunell and Gus Frerotte. And the Mark Brunell part is a compliment in my eyes.