I have recently purchased OOTP6 (that Out Of The Park Six for the uninitiated) and have become - unsurprisingly - completely and utterly addicted to it.
In the two days since completing all the needed downloading - the game itself, a good roster set, stadium pics... and an aborted attempt at a player icon pack... I have completed 1+ season as the Brewers. You might be interested to know that I got them into 3rd place in the central and a 90 win season in '04, while maintaining their position as #1 farm system in the majors.
On to the game, though.. my frame of reference is OOTP4, as I skipped 5, and that game gave me countless hundreds of hours of enjoyment.. and 6 is so much better. It's not even funny.
The in-game action is pretty similar. Few nice tweaks, no complaints.
What is great, though, is the new behind the scenes GM action. There's a 40 man roster. Waivers, option years, arbitration, Rule V draft. I can't describe how fucking cool it is outside of:
IT'S FUCKING COOL.
but you won't