It's not that Absolution is a downer, it's just too damn long. I can only play it straight through on rare occasions - it's a bit of a large chunk of pomp to bite off all at once.
If someone were trying to get into Muse, I'd recommend Black Holes or Origin of Symmetry. If they were more of a pop rock/mainstream rock fan I'd say BHAR, if they were a hard rock fan or metal fan I'd say Origin of Symmetry. OoS has some AMAZING melodic hooks, though, moreso than Absolution in my opinion.