Went to Stonehenge about 8 weeks ago. Nice trip - not to far from London; basically takes about 2.5-3 hours by train, then it's a 30-45 minute bus ride from the station. You probably want to go during opening hours, because they do have a fence cordoning off the henge, so you can't get too close without paying. Plus, it really is in the middle of nowhere, and you don't want to be stuck out there without a bus ride back - cabs are expensive (I believe the bus driver said it was a 30-35 pound cab ride), and I believe the buses only run when the henge is open to the public.
It takes about an hour to walk and do the audio tour. To be honest, I wouldn't imagine access to the inner circle would give you that much of a differnet perspective (you can get decently close on the walking tour). But I'd definitely recommend a visit out there - I certainly enjoyed it.