John Sickels' team-by-team Top 20 prospects lists are all done, and some analysis of his individual player grades puts Seattle's system at #13 in the league. Of course, the specific ranking means little given that Sickels himself says there's not much separating a C+ and a B- prospect, but what you can take from the rating is that the Mariners' farm system is reasonably above-average.
Scroll down, though, and you see that the organization only has four players who received at least a B rating. So, while it's got some depth in terms of live arms and interesting bats, there aren't too many impact players on the horizon. Which I suppose we've known all along, but now we have a little more support for that claim.