Actimel is produced by a company called Danone, along with a yoghurt called Activia. Danone claims that these products aid the digestive system and improve the immune system.
However, in 2010 Danone voluntarily withdraw these statements from France & the UK banned an advertisement campaign. After it was ruled that no evidence was found to support the claim that it helped to protect school-age youngsters against illness.
Actimel specifically is said to help your larger intestine which is where the majority of the 'bad' bacteria live. Interestingly one study looked into the ability of the bacteria in Actimel to make it through the digestive system. Only 51% made it to the small intestine and 28% was found in the faeces. This indicated that from the 10 billion bacteria found in one pot of Actimel only 2.5 billion made it to the large intestines.
Experts have indicated that we should really have 10 billion of each 
probiotic bacteria strain, of which there are 27 strains. This means that the 2.5 billion that Actimel provides really is not enough and maybe does not live up to it's claims.
Another point to mention is that it also contains high levels of sugar replacement which aids the living environments of the 'bad' bacteria, doesn't that defeat the object of taking it?
If you really want to benefit your gut then there are supplements that you can take from natural food specialists, which contain considerably more bacteria than Actimel. Of course you can also look at what you eat, food that have known probiotic effects are good quality sauerkraut, kimchi, cheese, natural yoghurt and kefir.
Personally, I think a balanced diet is always the best way forward. I believe that all the nutrients we need can be found in the food and drink we consume.