Well many patches (if not all) on J2O should be trustworthy. You're more likely to get affected by a virus from your email than a patch from J2O. If someone uploaded a bad program to J2O, it would get detected rather quickly, considering the number of people using the downloads. J2O isn't the only place to get a patch for jj2 tho.

There are many types of game patches available (registry patches, game executable patches, full mods, etc). I make executable patches myself and I always provide an option to make backups so that users may later restore the original executable, if something goes wrong. My suggestion is to check a couple of things to decide if a patch is trustworthy - the patch author and popularity of the patch itself.

You don't really need any patches for online play (except the official 1.20 to 1.23 maybe). Most of the patches are designed to enhance the gameplay or remove various game bugs. Although rare, there were cases in the past where a program would try to destroy jj2 or worse the operating system itself. These programs are mostly made by inexperienced individual programmers, who want to have some sort of revenge on another person. The programs then ended up affecting multiple users. If you're concerned that a program might do something bad to your PC, you can always consult users on this forum, or better even try the #jj2 channel on quakenet.

~Ol