I recently used the exact same roadbed on my new 4 x 8 layout. Curves are O31 and O42.
At first, I just formed the curves by hand, but found out that doing so in 3-6 inch sections works best. I tacked it down with small brad nails as I went. You really need a second set of hands to do this right. It seemed a bit wavy, but when the track is placed on top and then screwed down, the waviness goes away and it looks great. I'm very pleased with the final appearance and the sound-deadening qualities of this roadbed.
Plus, you get a lot of roadbed for the money. I'm glad I used it, and I highly recommend it.
Tom