I am 5'11", 240 Lb. man with a large frame, big boned as they used to say. I started wearing singlets instead of a T shirt and separate shorts about 4 years ago. I was having to travel to a very remote mountain weather facility to study climate change and severe weather events. Space is very limited while in the field, and even more so getting to and from the remote locations. Certain types of materials for clothing work better than others. These things are all one piece, fit very well, the 3XL fits a little bit loose, but I expected that. The 2XL is a comfortable snug fit and is great for support, quick to dry if you get sweaty, or it gets wet, and it is warm when it's cold outside, and cool when it is hot, generally that size I used for working outside or inside and you need to snug and tight to your body. I use the 3XL for mainly just laying about, down time, and to sleep in. It has the same quality as the 2XL, made of the same fabric, it just is not as tight on the body and yes these replace conventional underwear. They pack easy, any wrinkles just disappear when you put them on, and since it is one thing, shirt and shorts together, you are not hunting T shirts, or BVD's, and the washer is not eating one or the other like it does socks. This garment is made of synthetic materials so it is not suitable for some work environments, and you should be mindful of that. I would not put one of these on to work at Corning Glass Works in the glass blowing factory! I would say that firemen, police officers, pilots, steel workers, etc. Folks who work in jobs where there are high levels of heat, chemicals that can compromise plastic or synthetic fabrics, exposure to petroleum products like oil, and gasoline, are all in a day's work. They are awesome for camping trips too, just don't get too close to the campfire, embers do a number on the fabric! But hey, they can't be beat since there is no need to change in order to go for a swim. Just take your jeans off, and dive in! they dry very quickly so by the time you are out of the water 15 minutes you can put your jeans back on and move onto the next adventure! They are also very tough and resilient and stand up to pulling, tugging, and roughhousing! So, for me these just made better sense than the old T shirt and skivvies! Like I said you do need to be mindful of the material, and your environment, and what if any risks these could present in the absence of conventional thinking and mainstream cotton undergarments. Those folks may want to just keep these for weekend wear or for vacations. For me, they just make sense, and don't require a lot of fuss to pack, just roll them up and stuff then in a crevasse in your suitcase. They clean up quickly in cold water and don't require a washing machine, a sink or bathtub will do. While the label says cold water wash, delicate, I have found this to work the best. Semi-Warm, or Warm water, delicate setting, extra cold-water rinse, as in a second rinse, and do not use any fabric softener for either rinse cycle. They can then either dry on a clothesline, shower rod, or on the Ultra Low 60-minute timed cycle in the dryer, where one dryer sheet is mandatory to mitigate the static electricity and avoid static cling. As for laundry detergent I recommend using one that has antibacterial properties but is not too harsh. I have found that if you follow the manufacturers washing instructions and use only cold water, they can get a dingy look around the third wash as the cold water is of no benefit at removing body oils, and the accumulated odder. Woolite for Darks is fine, and the semi-warm, or warm water wash setting does a better job of getting sweat, antiperspirant, deodorant, and body oils out of the garment, as well as greasy food spills.