I am really starting to warm up to Rice as I hear some camp news coming out. I know that McGahee is still fairly young, but it's not like he's made of steel. He can get hurt too, and I would think that with his knee injury history, the chances of that injury being serious is pretty high.
I think that if Rice wins the primary backup and 3rd down duties, he is draftable in redraft and keeper leagues, and unless you really have a strong need for a starting RB in your dynasty, you might consider taking him above a guy like Kevin Smith or Matt Forte. Those guys may get the opportunity sooner, but they may not have the talent to take advantage of it. IMO, in a dynasty, if you have a solid group of starters, you should be focused on talent accumulation in your rookie draft, not situation.
But as far as situation goes, I like Baltimore's offense more than Chicago's or Detroit's (both long term and short term). The QB position there has made them look really bad for a really long time, but I think Cameron is going to get that sorted out enough for the offense can be effective this year, and can perhaps take a big step forward next year.
The only thing I am a little worried about that no one is mentioning, is that he carried the ball a lot at Rutgers. I think that is one of the reasons he fell in the NFL draft, and it may have had more of an effect on his draft position than his height had.