This is a very cool stadium. I live in Portland and one random weekend decided to make the trip to Seattle to see some sights and we stopped at Safeco to check out the Mariners. They were playing the Oakland Athletics.
We chose seats in 335 at the ticket office, we just wanted cheap seats near the infield and it's where they put us. (Row 12). Not a bad view at all, although if you're sitting down the third base side, you're going to be staring at the football stadium more so than the view of downtown Seattle. It's beautiful up in the 300 sections. I loved being able to hear the train whistles throughout the game, it added a bit of nostalgia to the ballpark.
The seats were roomy as well, no complaints there and our row didn't fill up so it was easy getting in and out to exit for the concourse area. The concourses were extremely wide and not even the least bit crowded.
The items at 'Cantina Nachos' were awesome though pricey but that's expected at every stadium and arena across the country.

If you're looking for the main team store, it can be found on the main level along the first base side. They had a huge selection of Mariners items.
Overall, I loved Safeco Field and I hope to check out a few other MLB ballparks to see how it matches up.