Background
The Théâtre des Champs-Élysées is a horseshoe-shaped theatre built in 1913 and famous for the 'Riot at the Rite' - the debut of Stravinsky's Rite of Spring. Nowadays it hosts orchestral concerts, recitals, dance and fully-staged opera; the baroque is a particular strength. There are around 1900 seats.
Location
The address is 15 Avenue Montaigne, 75008, a couple of minutes walk from métro station Alma-Marceau. Map and more access details here. The immediate area is expensive, exclusive and a bit soulless, though there some excellent restaurants and clubs nearby.
Booking
Tickets can be booked online here. Seats go on sale for the whole season around June, but there are still plenty available for most concerts.
The system doesn't allow you to choose your seat - it simply picks what it considers is the best available in the price category you choose. You can only see the seating plan after you start the booking process. Alternatively, you can reserve by phone on + 33 (0) 1 49 52 50 and pick your seats.
Cheap seats
Tickets are generally more expensive than in London, with top prices in the €80 to €170 range. Students can book seats for €10 under certain conditions, and a number of €5 restricted view places are available on the day (sold from 1 hour before the performance, but queuing starts earlier).
Best and worst seats
Seating varies from comfy armchairs with infinite legroom in the stalls, through dining-style chairs in the boxes of the corbeille (first level), to cramped ranks of doll-sized seats upstairs where no-one over 5'3" can sit in any degree of comfort. Upper levels are steeply raked; the corbeille and stalls less so. From the sides the view of the stage is clipped, as you might expect. All of the upper levels apart from the centre front corbeille are overhung by the level above, though this doesn't (in my inexpert opinion) affect the sound much.
I would go for the stalls for comfort, first row centre of any of the upper levels for view, and upper level side seats for value - providing you don't mind zero legroom and a partial view that is. The €5 day seats are in a gallery similar to the Royal Albert Hall's, and the only way you're really going to see anything is by standing and leaning out. Some of the most expensive seats are poor value, particularly the front extreme sides of the stalls, where both sound and sightlines are compromised. Once availability gets down to this level, you may be able to find better seats in lower price categories.
Dress code
Most people dress fairly smartly (this is Paris) but not formally; many have come straight from work. Jeans are fine.
Facilities
When the theatre was renovated in the '80s little was done to improve customer facilities. Cloakroom capacity is entirely inadequate - unless you get there early, you'll have to take your coat in with you. That's not a real problem on a cold day, as the heating isn't terrific either.
Whoever designed the loos wasn't expecting many customers either. Queuing obligatoire.
The bar selection is limited and doesn't include hot drinks or food. Prices are astonishing too - an orange juice is €4.50 for starters.
The second floor houses a small permanent exhibition which includes photos and models of the theatre - worth seeing once.
The marble entrance hall at the interval:
A close-up of the balcony bar. There's a similar one on the ground floor:
The upper levels. The gallery is behind those windows at the top:
Close up of the stalls and corbeille, with boxes at the rear:

