I've installed the PhocaGuestbook on 2 sites, 1.5 and now 2.5, and it works well on both. Installation and configuration was quick and easy. The reCaptcha seems to stop some spammers, but of course not all. There will always be those who like to spoil things.
I would not hesitate to recommend this component that I'm very pleased with, and the added bonus is that it's free. A big thank you for this guestbook!