From my experience :
flying the 757-300 is o.k., with a little lack of performance, the 767-300 is more fun, nice and roomy cockpit, easy to handle when flying manually.
Short/medium haul: work days can be very long, e.g. routes like HAM-TFS-HAM or DUS-HRG-DUS, 12-13-hour shifts. At airports with little or no night curfew you fly around the clock (especially annoying in the small hours of the morning). Night stops normally in Germany.
Long haul: there are nice trips, but as you start at the bottom of the bidding list you probably won't get there the first 2-3 years. Normally no augmented crew, so you'll be pretty worn out after the flight back, e.g. from CUN or MBO. Some trips are a pain in the a...,e.g. Seychelles. Sounds nice, but you arrive in the morning after a nightflight, and you leave again in the evening for another nightflight.
The working environment sometimes is "exotic" in every aspect (ground handling, ATC, crew hotels et al.). But if you are looking for a challenge then it's fine. And it is very nice to have a drink at the beach bar when you know that back home it's freezing cold...