Is the Lion USB drive purchased from Apple bootable?
Yes. You can boot from the USB drive. This means you can avoid having to get 10.6.8 and the Mac App store to download the Lion installer but can instead boot from USB and erase the drive and perform a bare install.

Things to remember:


Lion does not ship with iLife so consider pricing that or checking if older versions of the suite will work as desired.
Lion may not import settings from older backups as well as it does in Snow Leopard, but it's easy to try and erase and start over if problems arise.
You can always hand copy files and folders or make a bootable clone of the old system just in case you will want to boot the mac in the old OS to export some data you didn't know needed moving (old tax programs, etc...)