There are lots of negatives. Firstly, if you don't have a TESOL or Celta qualification you have to do a 120-hour course that won't necessarily have any validity outside of EF schools. You don't get a contract until you complete this. Fine, you may say, if you're just starting out in EFL teaching. After all, the course is free. Better to do a course that you can use to find work after your summer work is over though. In my case, I had a lot of experience teaching EFL but just didn't have the qualification (just a PGCE) so it would have meant 120 hours of unpaid hours for me just to get a 4-week job. Also, the company doesn't tell you what you will get paid UNTIL you complete the course. I did a bit of research and discovered that the company expects you to do 6 hours of teaching every week day and then take the students out on Saturdays for an excursion. They pay roughly £400 a week so we're talking about £11 an hour (not including prep time). Some people on here seem to think that's a good rate for a teacher. If that's your view, you're welcome to it. I personally found it insulting. If teachers don't push back against language schools like this, then they will always be treated as though they're not worth much. Don't work for a company that won't tell you how much you'll get paid in order to hide the terrible rate of pay.
