*raises hand* I've been to every Kumori Con since first year ('03), and every Sakura Con since their third year (which was in 2000). There are people who are more seasoned than me, but not many who live in the Springfield/Eugene area.
For a con attendee who has been to Kumori Con, here are the things to watch out for if you go to Sakura Con:
1. Lines are longer in general. There are more events, too, so it's not necessarily exponential, but longer lines are a fact of life for a "Big Ten" convention.
2. It's downtown, whereas Kumori Con is still being held at hotels which are further from the center of the metro area. This means that Sak's hotels are significantly more expensive, and they also get far more "walk-in" attendance now that they've been there for two years straight. Additionally, hotels will gouge you on parking costs. Expect to pay more than $10/night at most hotels. Always ask about their parking rates before making a reservation.
3. Huge crowds make it difficult to find people. You WILL miss some of your favorite cosplays, and have trouble finding friends unless you know where they are. ALWAYS exchange cell phone numbers, and be very specific about meet-up locations. And try to have a designated buddy if you hate wandering alone.
4. Since Sakura Con is held at a downtown convention center, it is no longer connected directly to a hotel. This isn't always a huge inconvenience, but if you don't get "dibs" on the closest hotels early on, you will have some walking ahead of you. Last year I had to walk 5 blocks to get onto convention grounds, which was especially nerve-wracking when I was doing a crossplay. T_T
It also means that you should learn the area as well as possible to avoid wasting your energy walking around the outside of the building to use the front entrance. The difference between the front and the rear entrance could be as much as 5 minutes walking distance. What's more, it's on a hill. o_o;
5. The first "official" hotel on their list usually fills up LONG before the convention starts. I haven't looked, but it's possible that it is already booked full right now. You need to have a ridiculous amount of foresight to avoid having to walk a long distance to the con, especially since the closest hotel is also the most expensive, and no longer directly affiliated with the con, so they don't offer a Sakura Con group rate.
6. Since it's at the gigantic downtown convention center, there's plenty of space in the halls, despite the fact that people are always moving in droves. The convention center has escalators, which are a GODSEND, combating both distance and multiple floors at once.
6. Main events are
huuuuuuuuuge. Prerecorded CD tracks are required for cosplay, microphones don't always work well when they ARE used, and you may have to resort to watching the TV screens to the sides when you can't see what's happening on stage. Personally, I hate ALL of the above and would rather just skip all live events with 1000+ attendees. After all, the quality of the skits is only marginally better than Kumori Con's, if at all.
7. On the upside, bigger events mean more fans of obscure anime and video games! ...provided you can find them. If you enjoy an obscure show or game, look hard enough, and you may be rewarded with some rare cosplays and fun conversation.
About registration: the reg AND pre-reg lines at Sakura Con in 2007 were the worst that they've ever been in the history of Sakura Con. This is because 2007 was the second year in which it was held downtown. In 2006, many people (mainly high schoolers) heard at the last minute that it was being held within the reach of public transportation, but weren't able to attend on such short notice. When 2007 came around, they were ready, and consequently flooded registration, resulting in a 2-hour minimum wait time which persisted from morning to dusk, even through Saturday and part of Sunday.
The bad news is that Sak' will be even huger from now on. The good news is that their staff now knows just how bad it will be, and will try their damndest to prevent the lines from being that long again this year. One staffer has already revealed one of their efforts to us (moving reg and pre-reg to the former dealer's room).
All-in-all, the main deterrent from going to Sakura Con is the cost. Every year I have to wrestle with the question of whether it is worth the huge drain on my bank account. Personally, I don't think being bigger really makes them any more fun than they used to be, but beyond Sakura and Kumori, there isn't anything else in the way of anime conventions in Washington or Oregon. Go to Sakura Con if you want to try it out, but bring your cell phone, pack a cooler of food, and plan your trip MONTHS ahead of time if you can. That means you should make your hotel reservations NOW. You can always cancel, sometimes the day before your arrival date! Just ask them about their cancellation policy, and you'll be good to go.