Been using this throughout the day today. So far so good. Lots of karaoke videos on youtube. Just search 'song title + karaoke' and you should be able to find most songs. Quality of videos so vary, some have lyrics, some do not, some have scrolling lyrics, and some have the highlight prompt lyrics (my favorite).
Don't actually need a smartphone or tablet for this to work. If you're at home, just power up the mics, and use whatever media player (PS4, etc.) you have to play YouTube videos on your TV. The smart phone and tablet only comes to play if you're in a emergency karaoke situation and no TV is present. Since AirPlay/Chromecast can be buggy at times, you might get dropped in the middle of a song.
Sing King on YouTube is the go to for videos. They have quite a few and decent presentation with lyrics. I did get tired of the BofA llama commercial and Samuel L Jacksons what's in your wallet promos that keep popping up every other song.
As for my go to song? Hmmmm, not a good singer, but I like After 7 Til you do me right and Ginuwine differences.
Grobans Prayer has a nice duet version on YouTube, also Eric Benet/Tamia Spend my life with you.