I've never flown an airliner, but I'm a PPLer and have flown a few proper airline full-motion sims in my time (737, Trident and A320) and they very different to a PC simulator.
PC sims are great for a bit of fun, as it's nice to push all the switches and start up the APU and bing the seatbelt signs etc, and some of them are truly excellent (the A320 professional add-on sticks in my mind) for learning all about procedural flying, SIDS, STARS, ILS approaches etc and if you've got a powerful enough PC you can get all the bells and whistles.
But, and this is the big but, a PC can't replicate the feeling of actually 'flying'. Certainly I know what a Cessna feels like to fly and the FS one is much, much harder to fly visually than the real thing because you're missing all the sensory clues that you get in real life. You've got that big panel in front of you, you fly in a 2D world, and even with a force feedback stick, the 'feeling' isn't there.