I don't work for TNT, but I may answer a few questions :
Liège does not look very attractive at the first glance (it can look gray and industrial, especially if you're from Spain), but it's a very nice lively city once you know it. It'll give you a lot of nice surprises, but you need to search for them. It's the 3rd biggest city of the country, so you'll find anything you need. The city is rather young and party oriented, while the suburbs offer green and posh neighbourghood (especially the east and the south, max 25min from the airport). Your net salary won't be high, but you have to take into account all side benefits and the high quality of life. Union is indeed a problem in Belgium, but TNT seems to do well there ...Brussels, Antwerp, Cologne are only 1h away. Paris, Frankfurt 2h with the highspeed train.
wiki in spanish:
Lieja - Wikipedia, la enciclopedia libre