Unless the plugin you’re considering is widely used and well supported, I’d think twice about using it. Chances are it’s got bugs or will not be supported in the future.
Case in point, Theme My Login and Search Everything for Wordpress. Both had bugs which required that I dive into their inner workings for some debugging and bug fixes.
If I ever notice Wordpress acting strange or features suddenly stop working, the first place I look is at the plugins. If you’re ever in this situation, try disabling all installed plugins and see if that solves the problem. If things start working again, try isolating the offending plugin by enabling all one by one. Buggy plugins can create small annoyances which can go unnoticed.
With that said though, there are of course really useful, well supported plugins. Take your time evaluating them and also consider building it out yourself if it’s not too intensive. As always caveat emptor :)