A couple of thoughts:
1) The walk to the bus station is very long and has some quite fun moments along the way (and 4 photos to be taken; three in front of the green screen and one in front of a lovely onion carriage). The entrance queue basically screams Harry Potter/Kings Cross. I dread to think how long the walk to the first show will take with groups - but they're doing groups of 35/45 - so that's quite good!
2) The 4D ride is amazing. AMAZING. The wrap around screen is very impressive and the fact that all the dreamworks characters make an appearance is fun. I wanted a bit more rocking on the bus ride - but I'd have to go again to properly experience it. My jaw was too much on the floor to really take it all in! Bravo SimWORX!
3) The shows are fun. They're quite dungeon like - but there's a stronger through story line in so much that some members of the group are given items to create a potion at the end. The kids will LOVE it! There are some jokes in there for the adults too - and it's a lot funnier than I thought it would have been.
4) The mirror maze is of a good size - with some nice magic mirror visuals.
5) There are some very clever moments involving a furball and an onion in two separate scenes - which really made me blink in shock! Great stuff!
So there are some thoughts. I don't know if it will merit multiple viewings as I don't see how they can update it in the future. Maybe have snow up at Christmas or something... or do shows based around Shrek The Halls or Scared Shrekless.
I'd be very interested to see how it will handle with large crowds. The fact that the 2nd time I went around with James... it was just me and James! It made for a very surreal experience! However in the first group - the kids' faces were just all smiles. This will be a huge hit for families and that's what the Southbank is for.
It's the best theming Merlin have ever done; the details are just astonishing. I don't know if that's Dreamworks' demands - but there's so much to look at! Smells galore (hold your nose in the first scene in Far Far Away!)
So - overall - very positive. Give it time - they're still working on some effects and moments. Make suer you stand in the centre of the vortex room (and if you've seen a video of Mystic Manor or the queue for Oblivion: The Black Hole - you'll know what effect to expect). I do with they could hide things like projectors, speakers and the emergency exit signs (though green) are still really garish in some places. Damn health and safety.
My gripe is that we see the other dreamworks characters in the first ride and they don't appear again until the Madame Tussauds style exit area.