Quindici,
The fact that there is a CFI in one of the driving seats does not end the "legal" issues. This has been discussed at length in previous threads, but as far as some regulators are concerned, it is possible/probable that they would not consider such flying to represent loggable P1 time for the person buying flight time - I am talking mainly here about the good old UK CAA and (these days) their interpretation of JAR-FCL and the ANO. Perhaps you are not particularly keen to acquire the experience as P1 time, in which case there may be no problem?
Don't get me wrong, I am not saying it has no value - it could be useful experience - but you need to be quite clear what you are hoping to achieve from it.
Lots have done it, and used it as a useful way of leaping one of the (many) hurdles to progress in this crazy game.
Probably the best thing to do is talk to the people who you are thinking fo getting work from when you have acquired those precious hours.