GG,
I have a number of friends who fly for FlexJet in the States. Most are very happy and thoroughly enjoy their jobs. The airplanes are high tech (Lear 31s, 60s and brand-new 45s, Challenger 604s and a fleet of Global Express aircraft on the way) and relatively well maintained.
The flying is very diverse and challenging at times - frequent flights into smaller, uncontrolled fields. Essentially, Flexjet has been described as a corporate job with an airline schedule because you actually know your schedule long in advance - typically a six-day on, four-day off schedule in the States. I am not sure how the European schedule would differ.
I have talked to a number of pilots who view Flexjet as a route to the airlines, and others who view Flexjet as the "ultimate" career option with great airplanes, great route variety, interesting customers, medium pay and significant job stability (due to the fact that the owner is the manufacturer).
I recently sat in the flight deck of one of Flexjet's Lear 60s - quite a nice view from the sharp end of that dart!
Good luck with the interview and keep us updated on your progress. Where would you likely be based - London, Frankfurt or Copenhagen?
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In the land of the blind, the one-eyed pilot is king!