Well, my guess is that it can differ from person to person, so I'll input in detail (as best as I can anyways) what it feels like to me.
(ahem)
Inflation has been something I have practiced on and off throughout the years, around 6 not inlcuding my 3 year hiatus. Throughout that time it has felt relatively the same to me, from what I remember more pleasurable as a teenager (due to obvious reasons).
To start with the inflation, I'll list both air and water, and both anally.
Inflation
Air: Air was something I think we all started off in, and it's something I like to go back to from time to time. With air inflation, you feel this sort of anticipation of whats about to happen when it first starts. Assuming you go via aquarium pump or another automatic device, you don't usually feel much for the first 5 seconds or so. After that you can usually feel a sort of pressure start to set in, like flatuence, and after more inflation, or stopping for a couple seconds, the air kinda just breaks down and flows into your intestines. This is where it gets pretty cool. As the air fills you, you can feel your intestines gradually fill, starting with the first section then towards the others afterwards. You can usually feel a bit of vibration as well, and each time there gets to be too much pressure (assuming its not your limit for pressure or capacity), you feel it build, then break into another section and you see your belly shrink a bit as well. As the time goes on you'll start to feel more and more pressure and generally a bit of pain, so that's the cue to stop. Often times duing this inflated state, you have a bit of trouble moving around (I haven't had this issue with water), and you'll probably just want to lie back and relax. It's common to feel a bit of cramping, but it generally isn't too sharp. Breathing abdominally becomes a struggle, and you'll probably have to take shallow upper breaths until you make some room.
Water: Water is my current favourite method (via an enema kit with an attached colon tube) of inflation (due note that if not done with isotonic water, you probably won't feel too good after taking one. Too much water gets absorbed and isotonic prevents that). The feeling of water in contrast to air is this heavier stretching feeling, almost like a section of your insides (pretty much the large intestines) feels like its thick with cake batter or some heavy and thick liquid ingredient. I find this to be quite relaxing, and if the temperature is perfect; I don't really feel it very much, just this growing wave of relaxation as more flows into you. Ironically, I find that I enjoy breathing abdominally whilst laying down doing this as opposed to breathing in the upper chest cavity, as your whole belly feels soft and almost solid, just like you can feel it so much more. There is generally a bit of tightness depending on how far you go, but assuming you do something midrange (2-3 litres) as opposed to 4-4.5, its both relaxing and filling/strecthy at the same time. I also enjoy the look, because where as air is light and pushy, water feels heavy and fulfilling when you stand up and stare in the mirror to look at yourself. You also don't feel restrictive for movement despite the amount of liquid inside you pushing on your body, like I said, its just like it's thick. You'll probably want to just relax and remain immobile anyways though.
A cool thing to note here is that I also have this device called a double ballon rectal catheder, and it's basically a device you plug into your backside and inflate like a balloon, so you can keep the water inside without any cramps from the rectum or accidental leaks.
Deflation:
Air: Air has both goods and bads to it, but the bads can just be for me. For the goods, the relief of farting after inflation with so much air in your colon is terrific, and you'll make the most amazing and impressive farts in your life, with the added benefit (for most people) of it not stinking much. Seriously, you can fart so loud the neighbours can hear it, and I'm not even kidding! it's also nice that you can deflate wherever you want, be it the bed or the living room if that's your go to inflation place, and there isnt a mess to clean besides the equipment used. Some of the bads, however, are things like cramping for the rest of the day, as the air has a lot harder of a time getting out of the body than water (things like internal fecal matter can block air from getting out), and I found that my anus gets quite sensitive if I fart a lot. Usually to the point of incredible stinging after I do it too much. Of course you cant stop though, so you have to fight through the pain like Rocky Balboa fought through Ivan Drago's unrealistic steroid punches (Rocky IV anyone?). For things like this, at the beggining, I like to leave the aquarium tube in so that the air automatically exits my body without me even trying (as there is more pressure internally than externally. This can be very dangerous though as the tube can puncture the rectal walls if you aren't careful.
(funny real life story actually. So as a teenager (aged 15), I had inflated one day in the bathroom. As I walked out my older brother came down the hallway and he saw me. Instinctively, I let loose, and let me tell you it was the louded, painfullest, and most epic fart I've ever experienced. It lasted about 15 loud seconds for the first fart, and me and my brother were both in tears from laughing. He didn't know I had inflated, or that I inflate, so it was a pretty hilarious moment. And I just could not stop farting. It hurt so much but it kept coming out with each of our laughs! He never did question why, and thought it was one of the top five most hilarious brother moments, the pain endured for that was brutal.)
Water: Water deflation is a mixed bag. For one, and I'll try not to gross people out here, the feeling of warm water rushing out your backside is actually super relaxing and if you have the temp right surprisingly pleasurable. It's nice because since there is so much pressure to go usually, the excretion barely takes any effort, so you dont have to strain or really try: It just kinda exits without the effort, assuming your not trying to keep it in. It's almost like eating the perfect amount of bran.
The cool thing I also enjoy about it (although is oftentimes a pain) is that assuming you took a midrange-high capacity enema, water can sometimes flow from one large intestine section to the other (water commonly stays in one loop and moves on its own with the body), so you can actually feel your colon gurgle and the water move through it. It's kinda like when your stomach rumbles but its not uncomfortable or signalling your hungry. The issue here is that the water is actually flowing into your rectum, so you'll feel like farting. The problem is that the nerves usually confuse the body temp water with air, so while it feels like a fart, well, let's just say that if you try to gamble it, I guaratnee your going to lose. And since its in your rectum and more water isnt coming, it generally stays there until you excrete. For the most part though, the process is fairly relaxing done right, and you get this sort of "empty" feeling in your gut. Like it's a lot lighter inside. The whole deflation process can take 5-10 minutes for the first amounts (the large amounts), and then 1-5 hours after for the pockets of water still trapped in your large intestine. Also, it can be very messy, so it's reccomended to take a shower afterwards.
I might add more to this in time, but it's a little late on my end here.
