So, I bought an inflatable plug a long while ago because I like the feeling of being stuffed full. It certainly manages to do that and also, as you say, has the advantage of giving you the full feeling without having to first warm yourself up to inserting a larger solid plug. That, however, is one of the main problems for myself. I found a major component of my desire for being filled includes the feeling of my anus being stretched to fit things in to begin with.
The inflated plug is as solid as you would expect a well inflated party balloon to be, and whilst it does put pressure on my prostate, it doesn't seem to stimulate me like a solid toy would. It feels as if the toy moulds itself to the shape of me inside rather than outright pushing against my prostate with no 'give' as per a solid toy. The material when inflated isn't as thick as I think most would expect; It is, as I said, pretty much like a party balloon and goes slightly translucent when inflated. I find the inflated part itself isnt stimulating enough for my liking, however, you can move the solid inner part of the plug around quite freely and manually push it against your prostate (mine vibrates- it is this one http://www.lovehoney.co.uk/product.cfm?p=27555) which is quite nice.
Pulling the toy out when inflated can provide some of the stretching I desire. Inflated, it is quite easily compressed and whilst it will stretch you to an extent, the air inside is squeezed to make the plug more elongated and it doesn't really stretch me enough. It also feels like use in this way could potentially rip the thing / pop it. You wouldn't really be able to insert it at all when inflated as it is too squishy.
I hope this helps.