Like anything it depends on the skill of the sucker, and whether the suckee finds it exciting.
I get more pleasure from my cock being pierced, in someone's unpierced mouth, that I do from the reverse. Although I do find tongue, lip, and uvula piercings enticing.
But, by and large, the primary point of a tongue piercing is that its a readily visible advertisement that you suck cock.
In speaking or kissing, its a continual tease reminding people that their cock could be in your mouth.
I knew a girl with a pierced clit who hung a little stainless bell from the ring. When she went commando in a skirt... you could hear that little bell ring when she walked or moved.
To most folks the sound was just a mystery...
But If you happened to know what was making the noise, knew where it was, and that that sound meant she was going without under that skirt... it drove you crazy with lust... you just could not get your mind off that woman's luscious cunt.
By the time you were alone with her and could get at her you were in a sexual frenzy... and she dearly loved a man in a frenzy of lust.
Similarly, if you happen to love sucking cock... a piercing let's anyone interested in on the fact and continuously teases them with that possibility.
In particular... for guys.... if you are a guy looking to get blown, guys with tongue piercings suck cock.