How important is the saddle on a TT bike?

I'm finally getting a TT bike through the team, and I ride a San Marco Regal on the road bike. Since they cost a f-ton, I might forgo that, and jump on the TT specific saddle we have at the shop I work at. It's a lot narrower{ed- in the rear, the nose is about the same), but the nose is well padded.
Since I'm on the rivet anyways, will it make a huge difference?

Also, how long does it take to get used to the TT position?