Ok, I went to the one in Cardiff in the Red Dragon Center (I think it's called that)..... It was actually very small and not much there apart from a few darleks and David Tennant's clothes on a headless model. I didn't think it was worth the money, personally.. maybe if you were an ABSOLUTE fantatic, then it wouldn't be so bad.
The Cardiff exhibition was good until the bigger ones came along.
If you are visiting Cardiff anyway, pop in for a look, but if you have to travel to any of them, then the Earls Court Exhibition is bigger. It runs until the end of the year.
There are other smaller exhibitions around the country, and a Doctor Who Museum in Blackpool (some of my artwork from the 1980s is on display there!)...