For years Blizzard has claimed that there would be no chance for faction changes. They didn't use the word "never" but they did say there were no plans on doing it. Much has changed since the Blizzard/Activision Merger. Well, the time has come, and what has become of it. I honestly can't tell you if the impact has been good or bad, but there has certainly been an impact.
If I haven't mentioned before, I play Horde. The first night that transfers came out, a number of my friends and guildies swapped characters over from Alliance. Good to see an influx of variety, but much attention turned to leveling forgotten alts. I don't know anyone who has transferred from Horde to Alliance. Given a better opportunity, I'm sure there are plenty who would make the switch.
Faction changes have had an interesting effect on guild on my server. A number of raiders from the top guilds swapped factions and joined forces with the top guilds on the Alliance side. The Alliance guilds have always been front runners in progression and this move has served to make them stronger. That's excellent for them.