Keep in mind that no system is 100% fool-proof, so the occasional visitor may venture to your site even if technically their country should be redirected.
Why? This is for things like using a VPN (a piece of software that allows the user to get an IP address from another country -- for example I am in Australia but I could use a VPN to pretend I am in the USA), mobile users in neighboring countries, and so-on. It will cover the large part of the user base, but it's always possible for people to circumvent IP addresses, no matter which system you use. This is probably only for a very small portion, e.g. very technical folks like me who use a VPN.
The rest will be redirected as normal :)