My usual experience with Rapidplay events is the regular Street Chess tournaments I run. It happens every Saturday and the average size if the field is around 16 players. Occasionally there will be a bigger field for a special event like the ACT Rapid Championship, where 30 to 40 players will normally play.
Today I'm helping out at the London Chess Classic Super Rapidplay. This is an incredibly popular event and attracts a field of over 400 players. It runs for 2 days, with a slightly longer time control of 25m+10s. One of the reasons for its popularity is almost all the strong GM's playing in the LCC FIDE Open back up for this event, and it gives everyone a change to mix it with the heavy hitters.
There will be a large team of arbiters handing the event, and it will be interesting to see how it all gets managed. So far I've only heard positive things about previous tournaments, so I'm guessing that the organisers have it down pat.