I'm on both Pentasa and Imuran, and have been for about 8 months. The only side effect I'm aware of was hair loss last fall, and that stopped when I added a B vitamin daily. My doctor thought at the time that it was my body's reaction to everything it had been through, and not drug related anyway.
My GI would just as soon see me off the Pentasa. He doesn't believe it does much. But I resisted when he suggested it, and he didn't push it. I was just starting to get stable at the time, and I didn't want to risk backsliding. Once I'm off the Pred and stable I'll consider going off the Pentasa.