They are great fun, but as everyone else is saying, Rush at Thorpe Park (which is unfortunately the only one I have been on) has a dreadful ride sequence, which is far too short. I'd say they are reasonably scary, if you aren't massive on airtime, as there isn't much to hold onto when you're at full swing. But it's well worth a try, the airtime can be fantastic, and completely thrashes Vortex in terms of swinging rides. Though I will admit, Maelstrom at Drayton Manor is the best swinging flat ride in the UK.
Just make sure you don't get stapled, that really ruins the ride.