I have some experience with using inflatables, and as others have said they tend not to stretch the sphincter. What happens is that they blow up inside while leaving the sphincter relatively undilated. Both my mine come from Denber Rubber. I have a small/medium one and the very large one. The small one can be inserted without difficulty but I need help to get the big one inside. It's too big to insert when fully inflated and stiff, but too floppy when not inflated.
The smaller one can be inserted to its entire length, but the large one is too long and inflexible for it to be fully accommodated in the rectum.
When my partner pleasures me one of the things that I like is for her to insert the small inflatable fully and then blow it up as far it will go, which makes it about 7 cm thick. I then like to push it out. This gives my sphincter a good stretch, but it can also cause haemmorhoids (piles).
Sometimes she will insert the big one and inflate it, but then she has to push against it to keep it in and in general the results are not as satisfying.
The inflation bulbs tend to slip off, particularly if lube gets on them. I have had to improvise new clamps on both.
Both toys are now over 5 years old now and the rubber of the smaller one has perished and become stiff so that it can no longer be used.
The Epi-No was originally developed from a device used by Ugandan women to stretch the perineum prior to giving birth. It was first trialled - with positive results - in Israel. It is said to significantly reduce perineal tear during child birth. I've never had one in my hands but I'm sure it would be a valuable tool for anybody interested in increasing their ability to take large objects in the anus.
You can read more about the Epi-No on this website http://www.epi-no.co.uk/