First, you have to ask yourself, do you want a firewall? It will be another application always running on your mac, and if you do not need or just are not interested in managing the internet connections of your mac, do not use it.
BUT, if you do, HO! is quiet easy to use. You will read about some stability issues (wich, in fact, are due to 10.6.8, and it is not developer's fault), but all of this is now solved with 1.3.2: if ever HO! should cause a panic, it will disable itself for the next boot in order to allow you to fix the problem (mostly upgrading to the next version, because they react very quick and are able to perform an upgrade in a few hours).
So, if you want your computer to manage you, don't buy HO!, otherwise, the software and the developers deserve your money and your trust.

Quite easy to use, it just does what it says, and it works good for me.

D