Meh. The reviews and audience response have still been good on the whole; the fact they weren't overwhelmingly glowing doesn't change that. I think our perceptions of the response are skewed by how much more extensively and passionately some of the detractors talk about it. That said, it's certainly true the movie hasn't been broadly deemed an absolute "must-see" by the general populace the way it did, say, The Dark Knight.
As far as the box office haul goes, it's pretty clear a fifth movie would probably still bring in many millions, but I think for purposes of determining its profitability it's perhaps more useful to think about its production costs. Indiana Jones and the Kingdom of the Crystal Skull, while certainly profitable, did cost an awful lot of money by all informed accounts - a hell of a lot more than the first three. Some of this is of course attributable to pure and simple inflation, but the fourth movie cost so much more than the originals that I think even after adjusting for inflation it's not only still the most expensive, but the most expensive by a considerable margin. I'd bet that if they made more of an effort to curtail costs, they could make a fine new Indy movie for considerably less than what Crystal Skull cost, and thus require far less of a box-office take to break even (or put another way, a movie with the potential to be a lot more profitable).