I really enjoyed it, though it certainly has flaws. I'm reserving final judgment, though, because Scott says the DVD/Blu-Ray is going to have 20-30 minutes added on.
It may not be a strict prequel in the sense that the events don't lead directly to the events of Alien, but it adds a lot to that mythology. And I'm okay with what a lot of people are calling "plot holes," because I think this is a franchise that benefits from some things being a mystery.
"COLONEL KLINK, WHY HAVE YOU FORSAKEN ME?!"