I also went to this at the Barbican a few years back (2003?)
It's a pretty great collection of games, from the early video games, all the consoles, handhelds and then some future interactive conceptual games (a precursor to a holodeck perhaps). It's more of a big collection than anything truly informative, and I thought the neatest stuff was seeing some of the original artwork and hand scribbled notes of some of the game designers. I mean, we all know what a Joust machine looks like-we don't really need to pay to see one, but getting a chance to see the behind the scenes rough designs is what really got to me.
I'd go again in a heartbeat.
The odd thing was that I swear I was about the 4th person in line when it opened on the day I went, and when I went in, there were already like 25 people inside playing games!