While all that stuff about B5 being not a mystery et al is true, my parents and a friend happened to watch some of it (my parents were recording it for me) and they all told me they were bored out of their wits and didn't understand how I could like that stuff. Of course I love it and came to believe that it is best appreciated by a fan who is familiar with the characters, allowing In the Beginning to act as sort of a photo album.
And, yes, the spoiler thing. One of the great things about B5 is that you get the mystery aspect if you watch it without any previous knowledge going into it and watch it all in broadcast order, and the then the "we know what's going to happen but they don't" thing when you watch it again. By hitting the newbie with In the Beginning, you're denying them the pleasure of the first.
I feel the first few episodes are strong on their own and don't see any reason not to just watch it in broadcasting order. As long as they can get over the "bird man" (as one friend called Londo, who himself did not get over it) and trusts you that there is a lot of sweet arc stuff, it should be cool.
Then again, I've never successfully hooked anyone on the show. Meh.
PillowRock makes a good point about how different things will grab different people. Look at my avatar. The Cartagia stuff is the first B5 stuff I saw. I was dying to know how that nutjob became Emperor and what was the deal with the orange guy being such a bad-ass.