S.F.
My son LOVED, LOVED, LOVED his preschool but also cried buckets shortly after he started attending when I dropped him off. That wore off soon enough. Then, he cried buckets when I came to pick him up! I wound up leaving him there much longer than he needed to be just because I knew he was having so much fun.
I got over the drop-off drama by walking to my car, waiting 5-10 minutes and then walking back to peek through the fence or classroom window. He was happily playing with his friends! It was gently suggested to me that he was feeding of MY anxiety about leaving him. I was startled by that since I was a happy working mom, but I guess I wasn't as ok with it as I thought if he could sense my feelings and I couldn't. Who knows if that was true, but it was certainly thought provoking. :)
Good luck!