My general policy on micro-transactions is that I'm fine with them so long as they don't provide some overwhelming advantage and that players can attain anything that can be bought with cash with a reasonable amount of effort. Emphasis on "reasonable".
Cash for Credits - If a player wants to spend a little cash to get some in-game money to buy that ship they want so they can play the game the way they want, I don't have a problem with this so long as a player without the spare cash to spend wouldn't have to spend an inordinate amount of time to earn those credits the hard way. This can be a fine way to keep the servers running, IMO.
Cash for Vanity items - I don't like it when micro-transactions give exclusive content to players. Again, I don't mind giving players the option to save time by paying cash that can keep the servers running, but any vanity item should be unlockable by spending time and effort getting achievements or doing specific missions as well.