I've done roundtrip LAX-SEA a bunch of times this summer and the flights were surprisingly full and I did not make my 1st choice on more than one occassion. The good thing is that with the high frequency, a bit of planning on your part can avoid most headaches. Once I had to use WN and change in OAK, but that's still not horrible. Check the loads and have 2 backups and you should be fine.

It's been said already, but I'll say it again- commuting to reserve sux.