I've been getting generic Asacol (really mesalamine or 5-ASA, to use the generic technical names) from Canada for over 10 years.
It has reduced my cost by over 50% and I really can't tell any difference between the Novopharm 5-ASA that I use (Novopharm being the Canadian pharmaceutical company) and Asacol (which is going off US patent in July 2013, by the way).
I am using CanAmerica as the pharmacy.
Other writers are correct, however, about how the pill's coating varies between different manufacturers.
So while the encapsulated Asacol/mesalamine has to be chemically identical the world over in order to qualify as a generic, apparently the pH-keyed *coating* does not - meaning that each manufacturer's pill may release its mesalamine in different locations in the digestive tract (based on pH variances).
I myself have found that I do better with the Novopharm/Canadian product than British/Indian products but I would bet that the matter varies by individual.
Try Googling "pH" and "mesalamine" - I believe there are a few internet pages that specify which pH each manufacturer's pills are optimized for. Your doctor might help with this.
Alternatively, you could try trial and error.