This is hosted every year by the Royal Society, is open to anyone who wants to go along, and is free to attend.
There are a number of excellent displays, where those involved in the project present their area of research. We have found them to be well presented displays with lots to look at, and usually activities to try. We have also had good experiences of people who are very happy to talk to us and to our children. My 8 and 9 year old boys had nuclear fusion, the structure of steel and how Richard III probably died explained helpfully and clearly to them yesterday.
In addition, when you arrive you collect a cloth bag which you can put various freebies in; we have a collection of sweets, badges and key rings. This is quite a fun way of keeping younger children involved, and helping older children to remember what they have been looking at afterwards.
Our children really enjoyed it. We were a bit concerned about our 3 year old, but she was actually quite happy to potter along, collecting bits for her bag and getting slowly covered in stickers. The older two enjoyed having things explained to them, and have both clearly learned some new science as a result.
Things to consider:
1) It was pretty busy both years when we went. You need to be a bit patient when waiting to look at a particular exhibition (though we were never waiting for all that long).
2) If, like us, you have fairly young children it is probably worth having an extra adult with you if possible, so you can split up or hold little ones between you. My husband has taken time off to come both years - which he is very happy to do as it is great fun for anyone interested in science.
3) If you go on the hottest day of the year, as we did, then it is a bit harder work!
4) We have been made very welcome with our children, but it is aimed more towards teens/adults. However, if, like us, you have younger children who are interested in science then it is a great way to further inspire them. I don't think it matters if they can't follow everything, or are only able to visit a small number of displays before they get tired. If they learn just a little more, and are further interested and enthused about science, then it has been a worthwhile trip.