Observation: I am unable to hear Ian McDiarmid on stage without thinking of Emperor Palpatine.
With my usual apology for not writing particularly coherent reviews...
E&G is not a great play. I doubt it's a great play in the full two-plays of five acts each veresion. It is wordy without the words being inspired, and as with a lot of plays with epic sweep, set in one place after another. It's Ibsen taking eleven hours, two world religions, and three empires to do what he would later do in two hours in a Norwegian drawing room. As the man sitting next to me said to his wife at the interval, "It's a radio play. They've spent a million quid to disguise the fact that it's a radio play." He was quite right*, and indeed I learn from Wikipedia that Radio 3 did an 8 hour version in 1990 with Robert Glenister. That said, the reason to go, unless you are particularly interested in little-staged Ibsen (which I am), is for the evening's theatricism. The Olivier auditorum's drum revolves, rises, descends, lights shift and blind, and my opera-loving soul enjoys a cast of fifty or so. In 3 1/4 hours (including interval) I might have thought "what the hell?" quite a lot, but I was never bored; unlike when I read Gore Vidal's novel on the same theme, which left so little lasting impression that I only remembered I had done so when Julian was explaining about his father's death and I realised where I had heard this bit of plot before. Julian himself is a thoroughly Ibsenian protagonist, self-aggrandizing, self-deluding, pitiable, dangerous, selfish, proclaiming strength when weak, pretending weakness when strong. It's no surprise to discover that Ibsen's next play would be Pillars of Society so much did red-haired Andrew Strong remind me of Damian Lewis as the latter play's Bernick at the National a couple of years ago. It's as if someone said to Ibsen, "Great stuff, Henrik. Why don't you try setting it in Andalsnes instead of the Roman Empire?"
Short version: highly recommended, but probaly best if you prefer a bit of theatrical sweep to Tom Stoppard (I have seen three Tom Stoppard plays, all very well done, and been bored and unmoved through all of them), and generally enjoy Ibsen.
*Though since the play was written to be read rather than performed, it's not really a surprise.