I love baseball and have been to most of the stadiums that current teams play at. Safeco is one of my very favorite. To begin with, there's truly not a bad seat in the house. The stadium was designed with this goal in mind, and it has been achieved better here than any other field I've been to. You can buy a $70 or a $12 one, and both afford excellent views of the field. Many Tuesdays throughout the season, they offer four tickets/drinks/hot dogs for $50, which is a tremendous savings. Prices don't go up on weekends, unlike most stadiums in the country (Wrigley's four-tier price system is horrible, for example), and--thanks largely to the Mariners' dismal record over the past three years--games seldom sell out. There's a TON of gourmet food on the lower concourse, including sushi and a lush bakery, so if you skip dinner you can fill up on good food. The roof is retractable so if it starts raining, you probably won't get wet.
Fans are allowed to bring in whatever food they'd like, but no drinks of any kind (go figure). The best hot dogs ever are outside the gates on East Marginal Way. After the game, they're two for $5.