From what I've heard, he is a great defensive player. Carney Lansford called him the best 2B he's ever seen defensively. It looks like he's made strong strides on his offensive game as well, and I fully expect him to be in the mix for the 2B job next season.
However, I don't know if he'll win the job. As ACS mentioned, there are plenty of candidates who will be vying for the position. If Stewart can make the transition successfully (up in the air right now), he might be the best option (he'd have the most potential anyways). Baker - not a fan of him, and I wouldn't be surprised to see him traded. The same goes for Carroll and Barmes - at least one has to be traded, and there seems to be mild interest. And then of course there are the free agent or trade possibilities, most notably Chris Burke and Mark Loretta, and to a lesser extent, Tad Iguchi and Marcus Giles.
I like Loretta, who has hit well at Coors, can play multiple positions, and it seems like he wants to play in Colorado. Burke could be worth a shot as well(rumors had the Rockies offering Barmes for him, which I'd take in a heartbeat, but Houston seems to want a starting pitcher and reliever, so a deal seems less likely). I read that Iguchi doesn't want to play in Colorado, and Giles is bleh.
So to sum up, the Rockies second base situation in 08 is very clouded right now with a large field of candidates. My preference would be Stewart, if he can make the transition to play an adequate fielder. After him, I like Loretta if he can be had a decent price, or see what Nix can do.