ECU/racing chips

I have an o2' OZ and I have a bad knack for shopping on ebay. There are alot of ECU/Racing chips for really cheap, that claim to add 20 hp or 35 hp. Should I trust them? Or could they make my performance worse? Is it better to go with brand name, more well know chips?

Those cheap racing chips are all crap on ebay... Somebody is out there just trying to make a buck on em...