This is a lovely theatre that I have visited on a number of occasions, most recently last weekend to see Bradley Cooper star in The Elephant Man. The original theatre was established in the 18th Century Georgian, although I believe the current building is from 1820 and externally it is a large and attractive theatre. I believe it holds 880 people. Nearest tube stations are Picadilly Circus and Charing Cross, so you have plenty of options for food and drink nearby.
I didn't book the tickets myself, but you can buy direct from the box office or online. Prices vary depending on the play and you can usually get good seats in the middle stalls or circle for £50 or less on some productions. This time we were in the Gallery (above the Upper Circle) for £13.50. The seats here are small 'benches' with arm dividers (rather than rests) and there is very little leg room. We had a clear (but high) view of the stage, my only comment would be that we sometimes struggled to hear some of the actors especially if someone coughed nearby and drowned them out. You are very high up here, and there are a lot of stairs to climb with no lift access due to the age of the building.
The theatre looked to be well maintained and didn't look shabby, but the lights are low.
There are a number of bars in the theatre - a vodka and diet coke and a bottle of beer was £9.40 which is pricey, about a pound more than a nearby pub. Sweets and crisps could also be purchased. Staff were polite and friendly and worked efficiently, so although it was busy you didn't really have a long wait. There are toilets in various parts too - for ye level they were up a further flight of stairs. They were clean but had a problem with water pressure so you had hand wipes instead. Expect to queue in the Ladies.
You can buy programmes from a number of sellers - for the last show it was £10 for a massive full colour programme, but I don't recall paying this much before.
Although this review is about the theatre itself, I really enjoyed the play and it is always a bonus when you get to see Bradley Cooper in his underpants :-)