I use SmugMug because my viewers do not have to endure the advertisements required to support "free" photo sharing sites. I also have a fair amount of control over the design and layout of each page and how my website is organized. Another thing I really like about the SmugMug viewing experience is the way my photos automatically resize to whatever size browser window / monitor the viewer is using.
I detest Flickr because of the popup windows begging me to "follow" you, and the advertisements shown after every 4h or 5th image. I won't look at a Flickr album because of the advertising, so I wouldn't ask someone to look at my photos there, either.
I don't attempt to sell my photos, but that is an option for those who pay extra for a "portfolio" or "business" account with SmugMug. All SmugMug accounts have unlimited storage. SmugMug features and account levels are listed <
here>
I don't use the "social" part of SmugMug, either, so I can't comment on that.
As a Lightroom user, one of the best features of SmugMug is the Lightroom plugin. It's quick and easy to create and update my SmugMug albums from within Lightroom. If I edit a photo in Lightroom, or discover I mispelled a word in the caption, it takes like 2 clicks to have the udates pushed to my website.
SmugMug is not the greatest for video. There is a limit of 20 minutes, maximum, for each video; I have made a couple of slideshows that went a bit over that, so I had to edit again, or break it up into multiple chapters. Additionally, the video player SmugMug uses takes a few seconds to figure out what kind of video quality to deliver, and during that startup time, the video looks crap. Not a good first impression. Once the video gets going, it looks fine.
It also takes a while to figure out how all the customization features work, but that's probably true for any website host which offers customization.