I didn't go with a guided group. I did, however, watch them carefully.
They seemed to know what they were doing. Beware; their success rate may be a little misleading: they kind of thin out the herd early on, shaking out the weak and silly before things get too serious. Lots of portly middle aged dudes are sent packing. You'll be with them for at least a month getting fit, more than anything. Lots of up, down, wait, up, down, wait, up.....Prepare for a long haul. Bring your sense of humor. You'll need it.
Read up on Denali trip reports for more. Good Luck.