Let me start off by saying that I actually like the prequel trilogy, so really, my opinion on this doesn't really count.
That being said, my thoughts on an Indy V are mixed. I watched the first two movies for the first time in a long while recently, and I enjoyed them immensely (I liked Raiders of the Lost Ark more, though). After watching Indiana Jones IV, my thoughts on it are a bit... meh. I mean, I thought it was good. I enjoyed almost everything about the movie up until the climax.
[spoiler] My main problem was with the alien showing up at the end of the movie. It just didn't seem right for an Indiana Jones film. Isn't Indy supposed to be going after ancient artifacts? The plot of aliens didn't really bug me until the end, with the alien showing up and then the UFO. Not to mention "The space between spaces" thing. Guess he was talking about Lucas' head these days. Also, I thought the monkeys scene was way too silly, but, of course, it wasn't very long and didn't detract from the film that much, if at all. Also, I didn't really like Mac's character. That's probably more of a personal preference, though.[/spoiler]
Aside from my complaints with the climax and the premise, I thought it was good.
An Indy V might be too much, though. I like Shia LaBeouf (I really liked him in Transformers) but I don't think he'd be right as the next Indy. In fact, there shouldn't be a next Indy, in my opinion. Harrison Ford is the true Indiana Jones, and having someone else replace would just cause the franchise to go downhill.
On the other hand, people seem to like Raiders of the Lost Ark and The Last Crusade more than Temple of Doom, right? Maybe the best Indy films only come on odd numbers. XD