Basically, the way I see it, Nintendo is at the very backswing of their momentum in recent years, which started with the N64's eventual market leading position loss and is now slowly being turned around by Nintendo. They've still got image problems, they've still got problems with developers, but compared to the N64, this is pure heaven. With the amount of respect Nintendo has been garnering with it's current owners and the (pretty damn good job) they've been doing with turning around their third party relations, it's only an amount of time before the GC starts gaining momentum. Anybody who sees Nintendo's lineup this year KNOWS it's even better than last year's, and anybody who likes Nintendo, really liked their GC last year, especially around last Christmas.
One thing a lot of people need to recognize is that Nintendo needed/needs to satisfy the existing Nintendo fans first and get all those games out that appeal to Nintendo fans. Nintendo's done a great job of doing this, and now that they've got that in full swing they can now start concentrating on making everybody else happy. So this year, Mario Kart,Metal Gear Twin Snakes, Final Fantasy CC, and Rogue Leader 3 are the "big holiday" titles... see a pattern? How about we add FZero to that, and Viewtiful Joe... now is it apparent? Next year it's going to be even more obvious. Nintendo's got strong 3rd party stuff, they've got "cool" games, and it's only a matter of time before their "kiddie" image starts to slip.
Nintendo can only gain momentum from where they are starting around August the way I see it, especially stacked up against the Xbox lineup. MS better have some serious surprises at E3, otherwise they're in trouble. There's only so long they can mooch off PC franchises before they relegate themselves to niche status, and without the help of Japanese developers, it's destined to happen, and I think it's going to take a lot more than just Rare to dig them out of that hole.
Right now I'm more worried about my Xbox than my GC honestly, especially now that it appears Sega is pulling a lot of Xbox support and moving it to PS2 while keeping their GC support about the same.