At first glance this guestbook looks to be good, but a little research reveals that the code is stolen from another script, which I cannot mention here. Stealing code and releasing it as ones own under the GNU Public Licence (GPL)!? -I can't support that, and neither should you!!!
As for the tech part, this script can give headaches to get running, and it requires register_globals to be "on" according to the install guide, which is not an option for many users hosting their sites on webhotels.