I just bought train reservations for the trip to Saarbrücken in December for the qepHom.
I haven't yet fully decided whether I'll go or not (this depends partly on whether I have the money then), but I thought I'd reserve seats well in advance. If I end up not going, it's not that much money down the drain, but if I decide on short notice to go, I'd rather not stand in the corridor for six hours at once or sit right next to the smokers' compartment.
The lady who sold me the reservations commented that especially for the way there, reservations are a good idea, since the 7:40 train to Berchtesgaden is generally pretty full.