In general its certainly not worth it. In general, by the time you actually take a head it would have been better to do all of that extra damage (+4d6 energy type damage and then toss something else on the mix).
On the other hand, it can let you take down the baddest BBEG with a single swing.
It was too strong in 3e because it unbalanced crit threats and multipliers. 19-20/x2 and 20/x3 are supposed to be equal (they aren't, but close enough), with vorpal they aren't. 3.5 version makes it balanced again, but vorpal should be cheaper (probably a +4, that way you can get it before its in the epic range of weapons category).
On the other hand, it can let you take down the baddest BBEG with a single swing.
It was too strong in 3e because it unbalanced crit threats and multipliers. 19-20/x2 and 20/x3 are supposed to be equal (they aren't, but close enough), with vorpal they aren't. 3.5 version makes it balanced again, but vorpal should be cheaper (probably a +4, that way you can get it before its in the epic range of weapons category).