It's a great game for anyone with a little more interest in business management games than just "I occasionally play Tycoon for a minute or five".
I've been using Airline 5 and 6 for almost two years for now, and have mostly good things to say. True, it's kind of a 'work in progress', experiences a few errors sometimes, but then again, the support is absolutely first class and if you have a problem you can be sure they'll find a solution for it. The latest, version 6.1 has just come out last week and it's quite thorough and sophisticated.
The learning curve can be quite steep for some people, but I would suggest it to anyone who has some interest in aviation and/or business management.
But to answer your questions:
Yes it is. Fully functional, very thorough and addictive in my opinion. The website
might look a little "amateurish" compared to some other software manufacturers (I must admit, when I first saw the site, I thought it was some kind of a scam to rip off aviation enthusiasts :P), but that's simply because the developers concentrate on the product rather than the accessories.
The basics are quite simple: you buy or lease an aircraft, configure it with seats, schedule it, advertise, set up service standards, deal with airport slot issues, etc. And stay in the black in the end of the day.
I consider Airline to be among my best 10 investments. As I said, it's interesting, thorough and addictive.