IP blocks are sometimes quite fragmented and country allocation changes from time. You should therefore use some accurate GeoIP database and have regular updates.

You could use browser language settings as an extra indicator to which country specific site to redirect. But just as with IP addresses, languages may appear to be quite fragmented amongst countries and you may run into problems in countries like Belgium and Switzerland whose languages are also main languages of neighboring countries.

There are a number of local issues in Europe like the small Belgian enclave in the Netherlands which in turn has smaller Dutch enclaves. (I think I have that the right way round). I don't know about mainland Europe but in England IP addresses are not a guide to local or even regional locations.

Piatkow is referring to Baarle-Hertog [en.wikipedia.org] and Baarle-Nassau. You won't solve that problem with IP serving though because the country borders are passing through some houses there and the country location of a computer may therefore change depending on the room it is situated in.

I use IP location detection for logging and ad-serving in Europe and from that experience I would say that the country information is about 99% correct. The main problems are with newly allocated blocks which don't have a country assigned in the database yet. Frequent database updates are therefore necessary.