We have been and it rather depends upon what you are expecting and your kids view of Lego.
We went at Easter hols and got there for 10am with a voucher. Two queues - one for turn up and pay and vouchers and the other for pre-booked guests. Seems a bit random as you end up at the same checkouts. You then get a lift up to the first floor and do the factory tour - which is a fella in a white coat, pushes some levers and knobs, tries to get the kids involved and you kind of see how lego is made. We got given a badge at the end. Doors opened and you then entered the main place.
The map is a little mis-leading - it is not as big as you think! There was a few lego creations in a Miniland type place - all north west things. Lovely to lok at but my kids had a quick look and then onwards. I think there are three rides - a forest pursuit one where you drive a lego car - that one is great, another one merlin and another that escapes me. There was a big pit of lego in the middle that had two pods that you could build on. All lego crawled over with shoes on so not clean!
There was another area that was lego racers - you create a car and race it down various tracks. That bit was quite good once you worked out to fix the wheels!
There was a cafe - didnt bother with and a girls lego area set out like a stage and beauty parlour - all pink and lilac lego.
Overall it was ok but we went with low expectations. We paid £10 to get in for adult and under 5 and DD has a Blue Peter badge so we got her in on that.
We spent less than two hours there and the exit is via the Lego shop!!