I've never noticed a problem with Splenda, Sweet N' Low or Equal, but I know of some people who don't have IBS but who are allergic to aspartame or sacchrain. There's also a product in health food stores/herbal shops called Stevia. Someone on the board here--pb4, I think--uses it without problems. It's from the leaves of a plant, and so isn't a chemical or chemical mutation of sugar like the other products. Personally, I switched from fake sugar to real sugar just because I figured fake sugar had to be worse from a health standpoint (not necessarily an IBS standpoint) than real sugar. Now I'm working on eliminating a lot of sugar from my diet all together.
I read one article on children and overconsumtion of sugar that quoted a pediatric G.I. as saying he felt that sugar consumption was linked to the rising amount of IBS and other digestive disorders he was seeing in children. Not a scientific study by any means, but something to think about. After all, we don't need sugar to live. In fact, it didn't appear in the Western diet until the Renaissance. Before the New World--and its sugar cane--was re-discovered, people sweetened things with honey or did without. By the Elizabethan period, sugar was all the rage and marzipan was invented and sculpting subtleties was the big crowd-pleaser at feasts (I went to a reenactment that had a period-correct life-size bust of Shakespeare done in marzipan). Oddly enough, people's teeth suddenly became very bad. Prior to cane sugar people in the middle ages actually had good teeth that became worn down over time, not rotted out. Honey happens to be an antiseptic and was used for a long time on wounds (still can be used on them if you're in a bind for medical supplies); I daresay honey helps the mouth by killing bad bacteria and germs in it. If you eat honey from a local source somewhat regularly, it can reduce your allergic reaction to air-born pollens.
I had a point here before I got into the history of sugar and honey.... Oh, never eat sugar alcohols and opt for real sugar rather than fake if you can and try and substitute honey for real sugar where possible because it's better for you (such as it goes). I have really found that the less sugar you eat, the less you want of it. I don't crave candy anything like I used to. Mind you, I ate some cake on Thanksgiving and for a couple of days afterwards I wished I had another piece or some chocolate or something, but now that I'm off it again, I'm back to not giving it much thought.