Well, I've still not ridden the new Link light rail in Seattle, so I don't have much I can say about it. However, on Sunday as we headed to the coast for day relaxing on the beach, we did see a couple of trains running along I5 (one north-bound, one south-bound) and both looked packed to standing room only. It may have been a weekend of free rides, but it looked promising. Reports are 92,000 riders over the weekend. I've still not seen ridership reports for Monday, but I'm confident that while reports are that they were low, that they'll grow as we get used to having light rail, and development happens around the lines, we'll see those numbers go up.
Seattle Tranit Blog has been all over opening weekend, and is following the latest on it.