It's one of the most themes I've played in SLB (disclaimer - I originated the concept for the league several years ago; a few different owners picked up the idea and started it in different eras).
But it is definitely not for the faint of heart. You get no SIM-generated AAA, no twists, no free agents...you use the exact players who appear for your team that season, period. If your team only has 1170 IP rostered, that's tough luck. The goal, as tigerfan2010 alluded too, is (for most teams anyway) not necessarily to win your division, league, etc. It's to improve, even just a little bit, on your team's actual record. Each season we redraft teams, and your draft order placement is more-or-less 100% determined by your performance relative to your team's actual W-L%.
The other fun stuff is being able to use all the players we've all grown up watching, seeing the real-life records accumulate (Barry Bonds is just about to cross 600 career HR in this league), having to use all parts of your roster, really learning how to micromanage PA and IP (tandem rotations are a godsend in this league), and reliving the actual pennant races. And it's definitely not a foregone conclusion as to who will win. This is our 23rd season - only twice has the real life World Series winner been champ for us.