I've been using it since day one pretty much, and although it was pretty buggy and unreliable early on, it's much more stable now, and I haven't had any issues fir a good few months. It runs very smoothly on my work PC (XP & Firefox), and on my mac & iPhone (syncing reliably, consistently).
I find the iDisk very handy for storing important docs etc., and I use QuickOffice MobileFiles Pro on my iPhone for accessing them on the go (recommend this app highly, there's a free version that doesn't have the email or spreadsheet editing functionality).
I've got a fairly complex setup for syncing, with my personal, race events & football fixture calendars syncing from iCal on my mac to MobileMe and my iPhone, and also syncing to google calendar (via BusySync) for an app I'm currently beta testing. I also sync my work exchange calendar to my iPhone and have had no issues with conflicts or reliability.
I was on the free trial initially which Apple ended up extending to 90 days in all after they had all the initial teething problems. Then I decided to stick with it and ended up paying for a family pack subscription, and am personally very happy with the service.
I get the impression that generally speaking, most users no longer have many issues, but there are still a few that still have problems. Check the Apple discussion forums fir more opinions.