Your life will become teaching and teaching will become your life. Not necessarily a bad thing if you are passionate about your teaching career, but you may forget who you are outside of the classroom. Do not mistake Teach First for a stepping stone into a corporate career; I would only recommend this scheme to people who actually want to go into teaching. Teach First are not supportive of those who choose to leave education afterwards.
Quickly gain a lot of experience, get paid while getting qualified
Long hours, very intense, no work-life balance in the first year