The portability really makes it easy to spend a lot of time gaming on the Switch. I have spent a significant amount of time playing the Switch during my lunch hour or whenever I find myself away from the house. Breath of the Wild, Mario Kart 8 Deluxe, and ARMS are a great set of games for the launch window to keep players engaged. It really is a huge draw to have a full TV-style console game experience while on the go. Also, Super Bomberman R is a blast to play with friends either on a TV or out and about. The NEO-GEO games and indie titles that I have purchased (Snake Pass, Shantae 1/2 Genie Hero, Tumbleseed, and Blaster Master Zero) have all been great to play as well. The Switch has a small but solid library of games, which is nice. The rest of the year looks good too with Splatoon 2, Fire Emblem Warriors, Xenoblade Chronicles 2, and Super Mario Odyssey all slated to hit before New Years. There's enough to play now and more coming real soon to keep Switch owners happy and gaming.
That being said, until Nintendo can snag a few popular 3rd party online games like Overwatch or something similar and few unique 3rd party exclusives, I don't see how it could be a total replacement for the PS4 or XBone. Plus, it's highly doubtful that big online games like Call of Duty or Battlefield will make it on the Switch. With a lack of big name titles from 3rd party publishers (online multiplayer or otherwise), it will be difficult for any entrenched PS4 or XBone owner to make the switch to primarily gaming on the Switch anytime soon.
Bottom line, I think the Switch is a fantastic system with a bright future ahead of it. Playing on the Switch has significantly cut into the time that I play on the PS4, and portability is a key factor that contributes to that. However, I've also been playing Overwatch and Friday the Thirteenth on the PS4 and will continue to unless they ever see a release on the Switch. The key to winning me over to playing primarily on the Switch is going to come down to the scope of the library both 1st and 3rd party, and how well their new online experience is when they finally roll it out.