I think signature campaign is may be the best way for people who are starting their new journey in bitcoin because it can helps them a lot to earn bitcoin in a fast way . I also suggest to sell service to other members like offering your programming skills or web designing and get paid on btc.
Signature campaigns are good but i will not consider it a fast way of earning bitcoins,it only provides some extra income which can be treated as a bonus,but yes if we have skills like webdesigning or writing,then we can work as a freelancer and take payment in bitcoins.
Signature campaign is a way of earning bitcoins and not growing them but if you can manage to save your earnings then it will surely take the amount of your coins to a higher level.