I've been on beta blockers for a couple of years now for migraines. I think the worst side effect was the nightmares for a month when I started them - I'd wake up in the middle of the room screaming. Mind you, I've always been prone to nightmares.
Do they work? Well I've only had a couple this year, instead of two or three every month - but that may be because I've been diagnosed as suffering from epilepsy, and it looks like that was the cause of most of my migraines.
The neurologist has suggested that when the epilepsy is under control that I stop taking the beta blockers and see if the migraines stay away.
[edited to add]
My cruising speed is down by about 10 or 15% since I started taking the BB's.