Altho I am only a novice RAGBRAI'r, I have never found a need for a sag, so I can't say if they are nice or not.
My first trip to Iowa was done in my 4th month of bike riding, so I was a long way from being what you would consider fit! While there may be days ranging from 60 to 100 miles (depending on your option, of course) typically, you ride between 10 and 20 miles before you pass thru a town, and have to get off the bike and walk anyway, so it's not bad.
And besides, the pass-thru towns are always a lot of fun anyway, so it's worth the stop, even if you don't need to!
So, don't worry too much about a sag, just get out there and ride your own ride, spin 'em up, and have a little fun!!