I've had an account with them since last summer but only really played since January. Initially I only deposited around £150 and within a month or two my profit took my balance to around £200. In the last couple of weeks I've invested another £600 into the index and my profit during this time has been around £25. So I wouldn't call myself an expert but I'm happy with the steady profits.
The price of players is only based on how many people own that player!
Here are some good ways of making money:
1. Regular buzz players (Zlatan, Rooney, Sanchez, Costa, etc). Quite simply this is the player that appears most in the news that day. Manchester United players are way more likely to win this than players of other teams. Quite often, the man united player that scores a winning goal will win the buzz that day.
2. Long term players. There's two varieties of these I'd say. Players like Alli, Sterling, Rashford, Kane who are young and will always play in the premier league. There's very little risk with these players. They might lose some value if they get injured but if you're happy to wait until they're back then you wont lose any money. The other variety of these is players are like Morata, Verrati, Andrea Belotti. Players that are likely to move to the premier league. These players are cheaper and if they move to the premier league their price will soar but there is also more risk. The moment news comes out about a new contract at their foreign clubs, lots of people will drop the player and you can get stung.
I would advice you to avoid the players who get promoted on Fridays. You have to have light speed reactions to be able to get them at a price cheaper than all the thousands of other people all buying at the same moment.
The premier league is the only league people care about on there so don't buy any players who could end up in China. Dimitri Payet was a solid player on there a few months ago and it only took days after his move to France and he was gone off the face of the game.
Hope this helps, I'm sure there's lots more I can go into if you need anymore. This was just off the top of my head