I run rigid on my Inbred 29er, Reba 80mm on my Rockhopper 29er and 120mm Maxle Reba on my Salsa Big Mama.
80mm feels like more than enough for most of my riding, i would run the Reba maxles at 100mm but they came at 120 and i couldn't be bothered to pay somebody to reduce them for me.
Although rigid with 2.55" low PSI tyres is fine for most riding (esp. uphill and flat) you still feel it a lot on some of the downs and the 80mm makes a big difference imho.
It's just difficult to find cheap 29er forks (like Tora, Recon equivalents) so you have to start looking at £300+ unless you get lucky on the classifieds/ebay. I do regret selling my Tora 29ers for £110 to a chap off here now – they could go on the On One but i didn't own it at the time i sold them.
Got my forks in the states and shipped over and despite the fact my folks are there at the moment and could bring me back a set, my chappy over there could not source me any Reba SL's (i wanted basic ones) saying that it seems the supply is dry until the 2011 models start to come through.