Last year I refurbished an old alum Gary fisher bike and swapped the Juddy TT pogo stick for a rigid fork. It was very fun to ride and taught me how to balance better and better handling skills. It definately slows you down, and you can't keep up with your biddues in squishies if you are gowing through a lot of rocks and/or downhills. The biggest downside is arm fatique over long rides.
For me, it was a cost effetive way to get into the sport and I am glad I did it.
If you ar eup for a challenge and don;t mind going a bit slower, go for it. Get a big front tire, get "loose" on the bike and have fun.
I now own a fully rigid steel SS 29er.....much different ride experience than an aluminum 26er. I have since put a squish on the front of the 26er.