In a way, Harrison Ford's been lucky, he hitched his star to George Lucas and Stephen Spielberg at about the same time that they were tearing up the record books - at one point he was in four of the top five highest grossing films of all time, and that's only because his scenes in ET were left on the cutting room floor.
But to be honest I think it was Lucas who was lucky when he got that guy who built the cabinets in his house to read some lines opposite actors casting for A New Hope. Star Wars would frankly be more than a little flat without Han Solo, and Ford has been performing the same service since. He's not an actor's actor, he doesn't really have a lot of range, but that's the point: his value is that when he appears on screen he is his own shorthand for a tough, likable, slightly dark character. Everytime I see him I think "Hooray, it's Harrison Ford! I like Harrison Ford. What's he up to this time?"
And there aren't a lot of actors that I get that off. I thought someone in Lucasfilm was being particularly canny when they cast Ewan McGregor in the prequels, as he is one of the few such out there. In the event, each shoot clearly featured George Lucas just off camera, holding a gun that he would raise menacingly each time that Ewan did anything other than a dry-as-bones impersonation of Alec Guinness. But I can see Lucas's worry - those films weren't going to suck their own ass.
What this is all in aid of, of course, is the casting of Shia LaBeouf as Indy's son in Indy IV. It's kind of a clear testing case for making more in the series without having to haul out Ford's carcass again, and Mutt* handily meets the requirements - he's tough but occasionally foolish, he can take a punch, and swashes a decent buckle when it's called for. He doesn't seem to have an emo bone in his body, LaBeouf would probably not be a good choice for Spiderman, but that's exactly what's not called for. More would be great!
One of the things I liked about the film is that it works because at its heart it works. Karen Allen's Marion is still a great companion for Indy, and Mutt a good sidekick. The scene where they start to work as a team is one that filled me with a sense of profound rightness.
Everything else is also good, unless you have a hatred of CGI animals for some reason.
*One thing that they're slightly unclear on is what his name actually is. The obvious "We called the dog Mutt!" joke is avoided, but he's named as Henry Jones III by Marian, despite the fact that it's not really the name she knew his father as. Also there's the fact that being named Indy would be quite handy for further films. On the other hand, her first words are "Indiana!" followed by rushing for Mutt, so maybe the "III" thing is a ghost of a previous script or something.