Personally I think Aeria hit the nail on the head with Twelve Sky, and TS 2.
They had some customizing gear, some ability resets, stat resets, things like that. The kicker was that they sold Chests that gave rare and possibly epic gear. But the chest didn't drop anything that you couldn't farm yourself. Also they had items for enchanting to better your success and stop things from breaking.
So it gave people that paid into the game a slight edge, but not something that overcame any non cash item people. Which personally I think is fair. Give an edge to those that keep the company going, but not hand the game over to the payers.
They had one game, you had a "Guardian" sort of thing but they were famous characters. William Tell, and others like that, in the cash shop they had healing pills that you could use every 3 seconds or so, and healed for thousands, best item you could get in the game would heal you for maybe a thousand and use it every 6 seconds.
That gave the game over to the payers. And as such.... the game is no longer up.
I think that customization, and giving an edge to payers is a good idea, but for any gear purchases don't sell overpowered items because it'll throw the game off, and the population will drop drastically.
For example. I spend 8-10hrs farming one really good piece of gear.... then again so I can combine, or whatever, and again and again.... and someone spends $5 and blows me out of the water... Guess what. I'm quitting.
I pay for items in the cash mall, customizing, enchanting, etc... but I don't pay for OP gear, I find it defeats the whole purpose of a game. I can drop $1000, and laugh at anyone who wants to fight me.
That's a game I play everyday for free... called "I Win". I always win.
And its cheaper.
I enjoy the game for what it is. A challenge to get gear, to level, to pvp, etc. Not to throw money at to win.