As with any type of "blind date" scenario - I've had some good experiences, and some horrible experiences.
Frankly, the horrible ones still haunt me so I have a momentary cringe every time it is suggested. But I try not to let a past experience ruin the opportunity for a new one.
Every one is different. THeir mood, their tone, their 'vibe', thier play style -- it can all vary. Also realize it will throw off your current balance. The new person may be very outspoken and overshadow other people, and so on.
It's good to make the first invite clear as a "sure come over for one game, we'll see how it goes" Ask the other players (email is fine) afterwards if they had any strong opinions. Unless the person does something terribly wrong, then invite him over to stay "i think that went well, we'd all officially like to invite you to continue joining us for future sessions." Or, if not, just point out that "it was fun/nice/etc meeting him and having him with you for the session, but we're not totally sure it would work for future sessions. some things were too distracting (or whatever the reason said in a polite way)
But, yes, as said, it can go right, or it can go horribly wrong. It's like rolling dice in a game or something...
