This name has a nice sound and a lovely meaning which is not obvious in Finnish (I hadn't even heard of the word before). It also doesn't sound dated even though it hasn't been popular for a while. Unfortunately though, Seija is very easily mistaken for many similar names (such as Saija, Eija, Meija or Teija), which might be annoying.
In my opinion, this name is the most beautiful of all Finnish names. Imagine how unique it would sound in America? I've never met someone named Seija. People should be more creative and this name is a great start. The meaning is just an added plus. =D