Yep if it wasn't for the online play I'd probably drive in a sim once every three months or so, if that There is nothing like racing against real oppenents. That is what keeps me coming back
LFS exceeds in this area. I've had LFS since 2004 and for a large portion of that time was stuck on dial up, then ISDN and have only just recently been able to get ADSL. LFS plays just as good on dial-up as it does on broadband the only limitation of a slower connection is downloading skins and the number of players on a server can't be too high.
Also the online features are so easy to use and conducive to not only racing people but to getting to know your opponents as well. You can join a server watch other drivers, ask some questions between races, share setups and be involved right from the first second you log on.