Downsides and Precautions with IDS

IDS will help you prevent break-ins but is not a guaranteed solution.
Be careful with Portsentry it can be fooled into banning the WHOLE internet very easily.
Snort will not protect you most of the time against very new exploits.
Always remember to update your Rule Sets and your running daemons (the newer they are less likely they have an exploit).

IDS will help you prevent break-ins but is not a guaranteed solution.