Yes, they take the spread.
Guess who gets the crappy end of that spread? Most forex firms, including FXCM, trade against customer funds for their income. They set the spreads however they choose. This is why their spreads are wider than ECN firms which charge a commission.
No, it is not in their interest for you to fail, but it is not in their interest for you to get the best fill either, as that is money out of their pockets.
I was not one of the guys that said you were nuts to use them. If you are good at what you do, FXCM is just fine. The problem is that you will pay a pip or two more than you need to usually, and occasionally get requoted, which is the worst part.
You will not get requoted using an ECN, and you will get the best fill available, usually via several sources. There is no conflict of interest as its their job to get you the best fill (an ECN takes nno position). All income to them is via commish, which depending on the firm, is as low as a quarter pip or so.
Trading with firms such as FXCM, Oanda, etc is fine when you are doing minis. When you do full size, it just doesn't make sense unless you are a position trader. Then it doesn't really matter. (except for security of funds).