I have two inflatables plugs: [url=http://www.premiumtoyz.de/erotikwelt/product_info.php/info/p313_Pump-up-Plug-large.html]dick-shaped[/url] and [url=http://www.premiumtoyz.de/erotikwelt/product_info.php/info/p5352_8---14-cm-----Pump-Up-Plug.html]ribbed cone[/url]. Note that my links may not link to the exact models: the dick-shaped is significantly smaller than the ribbed cone.
I first bought the dick-shaped one because I liked the size, the look, and I wanted to try an inflatable, even though I had been warned they were fragile and a waste of money. The warnings were wrong in this case! I enjoyed this plug very much, and still do on occasion. When it is barely inflated, its shape makes for a very easy insertion: it has a moderate width (about 1.5 inch), and doesn't go very far (about 5 inches). Ideal at the start of a session. You can then inflate it slowly, and feel it fill you. You can easily reach 3.5 inches, I haven't dared push it more than that (theoretically it goes to 5 inches).
I found many ways to play with it: inflate it a bit, and you get a small dildo that you can insert and remove easily, doing some strokes, etc. Inflate it a bit more, and you get a small plug that is as easy to insert and remove, with more sensations; it won't stay inside you. After that, you get a plug that will stay inside you unless you really push it; it will stay inside without bothering you body, this is a great way to train your rectum. Even further, you reach a point where your body really wants to eject that big log. Fighting your urge is also a great pleasure and training.
I bought the second one because I wanted to try bigger things, and I thought the ribbed texture would be interesting when inflated a bit. Big mistake: all textures disappear quickly when inflated! I also discovered that it was too long for me, I initially could not insert it up to the neck, so the cone could not stay in place, it did not work as a plug. Recently, I've managed to insert it fully (including the base), and I started liking it more.
One thing that bothers me with both models: once the plug is inflated and in place, the hose and pump are a bother if you want to walk around.
One tip: if you just press the pump, it inflated very slowly. If you plug the exit hole when you press the pump (with your finger, or by pressing that hole against your thigh), it inflates much faster. I suggest you keep count of the number of presses you do, so you know your progress, your limits, and have an idea of what is happening inside you (you sure don't want them to blow up!).