As i know, one from the longest running, signature campaigns is Bitmixer. Probably no one has such a good reputation as they have.
What does signature campaigns have anything to do with the type of service they provide? They can run a signature campaign for months and yet run a fraudulent service.Take betcoin.ag for example.Also Bitmixer is weeding out spammers from their signature campaign,make sure you aren't one of them. 
not much.
but also it shows that they have been around for over 2-3 years as far as i know and it also shows that they have been running a profitable operation that it allows them to have an active signature campaign.
if it wasn't successful or shady they would have stopped it long time ago.
I agree that having been in the business for so long means something. Especially when it comes to bitcoin. I do think however that in the end time is not the only factor to be taken in consideration. There are some new mixing servers which deserve chance too. Crypto Mixer for example seems to be having a very solid server.
We can never be sure of one's true intentions but those party's are for sure to be trusted.