I did one a looong time ago when young. I found it very interesting - thought the guides did a good job - I felt I had learnt new things; it brought history to life more ... although I would have liked more time to browse. Of course, the tour was taylored to 'Bible prophecy' and the WT spin on it and I didn't know at the time their dates were all screwy. I'd get so much more out of visiting now (with a non-JW tour guide).
Ah ha, there's a JW tour company who specialize in it: http://www.meander.co.uk/british.php