Occasionally they sell deals where the cost isn't bad for the end user; e.g., HP's Flight Fund to US' Dividend Miles right after the merger announcement. There was no cost to move from one to another.

But most of the time it is a rip off. I did the above and they ended up billing me $50 this January as part of an "annual membership fee" that they never alerted me to. So I disputed it with Visa.

Watch these guys, they are not to be trusted!