Have done it for 2 weeks once and, for a week, a few times.
Its generally been fine however we've been fortunate to stay in the sort of place where we've had space. I couldn't stand it if we were all crammed in somewhere without our own rooms ( a room for them, a room for us, a room for the DC) to disappear to. In the last place in France we were even lucky enough to have 2 living rooms on top of a kitchen and dining room so plenty of places to escape to for some time-out!
Also they don't mind us disappearing off with the DC for a day out. They don't expect us to do everything with them day-in, day-out.
They also give us time-off by babysitting whilst we go out for a meal which is really good of them.