They are awesome indeed but I read the helium lasts for only 6 hours!
So you really need to refill it often.
The deposits for refill get you up to 6 refills according to the instructions and, if I don't recall wrongly, they were about 35 Euros in Barcelona (around $47). So it is not VERY expensive, but it is not cheap either. Especially if children start messing around with them and if they want to play with them very often and for a long time...
We were actually going to buy 3 (one for my son, one for two nephews on one side of the family and one for three nephews on the other) for this Chritsmass, but I talked myself and my wife out of buying them because of these reasons...

Cheers,
N.