I'm a feeder, not a gainer, but I have put on over 100lbs since I stopped living on drugs and sadness (am now 230lb at 5'5" tall, UK size 20).
Pros:
I don't feel the cold anymore - used to spend all winter wrapped up in big jumpers and blankets and still shivering, it was horrible. Now I'm very rarely uncomfortably cold.
I don't get sick - my body is well-nourished and stronger because of it. I eat around 3000kcal a day and my body has plenty of energy to fight off illness. My personal opinion is that most people (definitely most women) don't eat enough.
Less catcalling! I used to hate going out alone because of the unwanted attention from men - now, almost nothing. If I didn't have brightly coloured hair and a skull mask no-one would even notice me. Also, the few times I have had a bloke say something disgusting, I've marched up to him and asked him who the fuck he thinks he's talking to, confident in the knowledge that I likely match or exceed him in weight and strength. Couldn't have done that as a small, weak person (disclaimer: I do a lot of strength training and my job revolves around lifting stuff, but just being larger and taking up space is a huge confidence boost as well).
Built-in cushioning! I used to sleep with a pillow between my bony knees because it was uncomfortable to have them touching, and my hipbones made lying down awkward as well. Now I flop and squish and can get comfortable anywhere, with plenty of lovely soft homegrown padding. Also give good cuddles, hugging thin people grosses me out a bit, it just doesn't feel nice and I'm afraid I'll break them.
Cons:
Being bathed in sweat on hot days. I can't cope with the heat like I used to.
Finding clothes that are affordable, fashionable, and fit well? Good luck with that.
Constantly knocking stuff over because you're a lot wider than you think.
Small chairs at restaurants and bars. Particularly those awful plastic ones with arms that cut into your sides.
People assuming you're unfit/unhealthy/lazy etc because you're fat. Also, people assuming I have type 2 diabetes and brought it on myself by eating too much. Jokes on you, suckers, I'm type 1 and have been diabetic all my life.
No matter how flexible you are, sometimes your belly just gets in the way. A lot of public toilets are tricky to navigate as a fat person, and my shower stall seems to have shrunk ๐ค