From GM POV, I think it can be very difficult to successfully integrate a new player & PC into an ongoing campaign. In my last campaign where new players frequently joined I never really managed to do this properly and players were several times left with unsatisfactory PCs and/or not understanding the campaign premises. This time round I tried to establish the player group in advance of starting the campaign, my intention was that I had 6 players and would not take more players unless most of the original group had left. I'm running Lost City of Barakus, a combat-intensive Necromancer Games module, in my warrior-centric campaign world. As it turned out, of the original players only one player played a dedicated warrior type, and that was after we told him the group needed one, he had initially wanted to play a Druid. I don't think he was very interested in the character - he already plays a legendary high level (18th) warrior in my occasional high level game, playing the same character at 1st level didn't much interest him and I think he only ever came for the first session. So I decided I'd be willing to take a new player, but most PC types would step on the shoes of the existing group; really I only had a vacancy for a warrior-type PC. So when a promising prospective player applied, I told her exactly what I was looking for, a warrior. She was keen on this, she wasn't into playing a knight like I suggested, but we discussed it and the character that emerged was a professional soldier veteran mercenary ex-Legionary now part of an Amerindian type tribe - kinda Dances With Wolves. The important thing was that the character was one I, as GM, found very cool and who I thought would fit well in the game.
So, I think my advice to other GMs adding new players with new PCs is to have a clear idea of what the campaign & group needs (in this case a warrior), make sure the player is willing (and seems able) to play this sort of character, then work with the player to develop a character (not stats, personality & background) who fits into the setting and, most importantly, impresses both player and yourself with their coolness. That last is very very important, the best bit of advice in Ron Edwards' Sorcerer & Sword. If a character doesn't really immediately grab both player & GM as a cool protagonist in their own story, there's something wrong. The GM needs to then communicate with the player - either the PC can be altered/adapted to become suitably cool, or the idea needs to be scrapped and start again. Life's too short for dull PCs.
Edit: OTOH if you run a disposable-PC game like Gygaxian D&D then players should be discouraged from investing too much time in creating their PCs; Gygaxian PCs become cool if they survive where the others didn't, they start off as disposable cardboard. Gygaxian D&D still has strict requirements though - eg the PC group needs to be well balanced, PCs need to be competent/min-maxed in their area of expertise, and so on. All this needs to be born in mind when the GM is communicating with the new player about creating their first PC.
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