I just jumped in (am still in my Entry League) and am hopelessly hooked already.
New packs to get new players, Auction house as you build up in game currency to get players you specifically want and/or fill holes in your lineup and staff and in the background the OOTP sim engine running ~ 30 games a day. If Football Manager is ever able to execute on something similar I'll probably be forced to quit my job and figure out a way to survive on Perfect Points
My team is the Raymond Landlocked Salmon and it's been a treat to build. At the 20 game mark a week into May we are 14-6 but on a three game skid. I used the Auction house to "sell" off some cards I got in packs that were highly rated but that I didn't want and have used those points to put Warren Spahn and Johnny Sain at the front of my rotation ("Spahn and Sain and pray for rain") while my offense is built around a hot start from Andrew Benitiendi in LF/CF and Terry Pendleton at 3rd who is scorching right now (hitting .500 over the last half dozen games with 3 HR including a walkoff.)
Check it out, it's free unless you want to buy more of the in game currency for the auction house but your team generates that anyway.