Market makers are forex brokers that provide liquidity to the market, always taking positions opposing those of their clients. The fact that they are counter party to a retail forex trader's trades in and of itself is not a bad thing, because occasionally they offset the difference between their net short and net long positions in the interbank forex market. In other words, these market makers hedge their net exposure in order to protect themselves from risk. Such forex brokers are generally reliable and would not easily afford to tarnish their reputation for the peanuts they would make if they start gaming you.
Usually market makers offer fixed spreads and your orders will get filled duly by the WYSIWYG (what you see is what you get), i.e. the prices you see on your screen - there's no slippage in calm market conditions. However, during high volatility periods (for example, before and after the release of important economic data, speeches by central bank officials, etc.) slippage is inevitable and is considered normal in the forex market.