Anybody using O2 Broadband

Any problems
I`m thinking of jumping ship from Demon and there is a good deal going on just now. Top up an O2 mobile at a tenner quarterly and it works out at £7.60. First three months free and another £50 cashback through my internet cooperative for signing up .
Works out at roughly £140 for 15 months minus £50 cashback ......equalling £6 a month .