I've heard mixed reports from my running friends about Vegas. Some thought it was poorly run, while others had a better experience. I know New Orleans gets good reviews, as does Edinburgh. I'd stay away from Madrid, though. They didn't do a very good job with that one.
I ran the New Orleans one last year and thought it was great. I ran the half and My friends ran the full, and we all had a great time. New Orleans is a fantastic city, and makes for a great run-cation. I want to head back there again in the next few years and do the full myself.
I've run Seattle and Portland the last two years and liked them as well. I would never do the full marathon again in Seattle (wasn't a fan of the course), but am signed up for the half for next year. The Portland race is perhaps my favorite, though that might be because I love the course, and the weather has been perfectly cloudy the last few years. I ran the LA one last year, as well, and that one was fun because so many people were in costume, since it was right before Halloween. I ran the Dallas one this year, and it wasn't that memorable, to be honest, other than it was shockingly cold. I liked how the last bit of the race looped through the park of the finish area, because it made it easy for our friends to wait for us and cheer us on.
The Vegas one has been controversial, as far as I understand. My friends ran it two years ago, I think, the year people got sick from the water, and didn't like it. I think it was in the day at the time. I ran it last year, at night, and enjoyed it for the most part, though there were a few dead miles where we were running through residential neighborhoods where they couldn't blast music.
I never realized they bused people to further up on the course if they fell behind. Do they still get a official finish time? It doesn't really effect me, because when I'm running, I'm racing against myself, but It just seems kind of weird that they wouldn't just do what Disney does, and bus to the finish area.
