I take the bus up there, so while it is grossly inconvenient I'll go when it is nicer out so that the waits for the busses aren't SO bad. Also the food at the outlet mall itself sucks and I ain't eating McDonalds. Granted, I have a bus pass, so I can pretty much go up and I just have to pay up a quarter each way for the 510...since I have the $2.25 Pugetpass. It's fairly simple though-if you are coming from Seattle, you catch the 510 downtown (Sound Transit), then you go all the way to Everett Station. There you can catch the 201 or 202 which takes you to Marysville. If you get the 201, I believe you can ride it all the way up State to the train tracks, and then wait for the 221 or 222. If you get off at State/Grove, there's a Safeway there so you can get a bite to eat and there is a Starbucks...since practically nowhere in the outlets allows food or drink (get with the times people...I work at Northgate and almost every place allows food). Anywhoo you get off of the 221 or 222 right at the edge of the parking lot...you have to walk further but it's really no biggie.
Oh yeah, total cost to me to go up there...50 cents plus food, because I have to have the bus pass anyways.