Dave,
I don't have much experience with Pentasa (I stopped after a few weeks), but I'm on Apriso now (my doc also recommended it after finding inflammation in my small bowel). Of all the 5-ASAs I've tried (Asacol, Pentasa and Lialda), I've personally had the best results with Apriso. I noticed an improvement within 24 hours, which I thought was a fluke until I heard of a few others with similar results.
The effectiveness of each of the 5-ASAs is dependent on where your inflammation is...with the exception of Lialda (which is a time release verion of Asacol), the key difference between the 5-ASAs is where in your intestine they are activated. If you're not getting results from Pentasa, it might be worth giving Apriso a try, you can always switch back to Pentasa if you decide that it was helping more.
Good luck,
Melissa