Hi Matt,
I used to work for Monarch - you'll do a mixture of longhaul and shorthaul (if you're based in Gatwick or Manchester.)
Expect a longhaul trip every other month, and if you're lucky you might get trips on consecutive months. Most trips last a week too!
When you join the shorthaul fleets are split. You'll either operate the A320/321 or the Boeing 757. If you're on the 757 you may also be trained on the Boeing 767 which does longhaul. However 757 or A320 - everyone is trained on the A330 which will guarantee you trips. Also most crew are trained on the A300 which is a short/medium haul widebody aircraft
The A320 fleet tend to do more scheduled services to Malaga, Alicante, Lisbon, Grenada etc, while the Boeing fleet don't seem to have as much shorthaul to operate. This does cause some friction within the company. Also a lot of the Boeing fleet are trained on two longhaul aircraft (767 and A330) so inevitably get more trips! I must stress that not all the 757 fleet are on the 767, as there is only one 767.
So to summarise -
Good points: Great trips which bring good money (on a week away you'll get about £500 traveller's cheques. What you don't spend you take home.), Friendly crew, some interesting "adhoc" charters (often football flights), permanent tan, smart uniform, generally good pay comapred to other airlines.
Bad points: Friction between Boeing and Airbus fleets, being charter the seats are crammed in therefore continuous complaints about legroom, A300 and B757 are getting quite old now but great aircraft all the same. Monarch Scheduled is becoming more "low cost", some flights operate a "meal option" service where passengers order a meal prior to the flight, or decline it for a cheaper ticket. There is always a discrepancy and a lack of meals on these flights.
Hope this helps, I had a great time in the company. The reason I left was due to slow promotion.
Good luck!