For a start, if you try googling information about this bus, it's blessed hard to find! So I've put the link in straightaway.
So, for all you people wondering whether to do Stonehenge by coach tour or DIY, hopefully this will help with your decision-making process.
Trains from London Waterloo are about half-hourly and take about 90 - 100 minutes, Mon - Fri. Personally I'd avoid trying to do Stonehenge on a Sunday and possibly Saturdays, as this is when most of the engineering works go on. At least check with national rail before you decide, to see if your journey will be affected. Certainly there were major engineering works today. Cheapest price per person return is £26.80.
You can then pick up the Stonehenge Tour Bus from either Salisbury Station, or the Bus station in the town centre. It's hard to miss, as it's a double-decker, completely painted to look like Stonehenge on a red day. (You can't miss the bus stop at the bus station either. It was 'B' stop, with a huge Stonehenge picture (same colouring as the bus))
You pay the driver (I didn't think to ask if he takes credit cards, sorry!) and it's either £11 just for the bus or £17.50ish to include entrance to Stonehenge. Buying the combined ticket doesn't give you a discount though.
Sit upstairs on the left hand side if you can for the best views. I was pleased to note there was a recorded commentary - in English only - pointing out various things of interest on the way. We went past Old Sarum and through Amesbury. If we went past Durrington Walls, I don't recall hearing that mentioned. Oh well. The commentary is fairly basic but accurate, and 20 minutes later we were at Stonehenge.
If you have the combined ticket, the driver asks you to wait in a group, and then you all go off to the entrance together to be 'fast-tracked' through. I have a National Trust card, and waiting in the normal queue didn't take long either :-)
The bus leaves about 10 minutes after it arrives - so assuming you're not staying on board, that gives you about 70 minutes to look around, visit the shop and have something to eat. It's just about long enough. I took an audio guide around Stonehenge, but was so busy chatting to the rest of my TALF buddies, I forgot to listen to it, so I'm not sure how long it would take to go around, listening to the tape. As it was I felt I only had just long enough.
The return journey is via a different route which made it more interesting.
Is it worth £11? I'll leave that to the Maccas37 who accompanied me to decide! I think it's a tad high, but then again, they are offering a good service for tourists.
Oh, on the way back, they stopped at Old Sarum, and if you wanted to get out and walk around the old castle/hill you could, and then just catch ANY Salisbury & Wiltshire bus back into town, so long as you had your Stonehenge Tour ticket. Don't have to wait for the special bus.
So, for a day trip from London to Salisbury and to Stonehenge with entrance, you're looking at around £44 per person. You need to compare that with the London Coach Tours and decide if you'd rather pay £20-£30 more, and see more places, but have short amounts of time in each, or whether it's better to have longer in just 2 places, but for less money.
But it is easy - and enjoyable - to do this day on your own and without a car. Just don't do it on a Sunday, if you can help it!