Hello ,
I have seen a lot of people using the smart CPM with decisive and I'm wondering if this is really good , someone can explain me how this is works?
They actually changing your CPM by the conversions you get? because it is not really smart if you have good campaigns with high profit and conversions they will raise your CPM?
Thank you for help 
Basically it will gather some data first ( conversions ) and will bid the correct CPM for each site to reach your target CPA you enter.
It's reasonably smart.
These days I use it mostly as an initial fishing tool - because it's got more flexibility to optimise it's easier (after some data rolls in) to identify opportunities for break-out campaigns.
so basically the best method is to start with smart CPM to gather data fast and then after you optimise the campaign to change it to CPM?
Yep use SCPM as a starting point. But don't expect it to auto optimize your campaigns to profit. It will give you some rough ideas how you should break out campaigns and bid manually.
the main issue is that once you find the right CPM for each source you need to create many campaigns because you can't set a different CPM for each source within the same campaign yet.
waiting for looong minutes everytime you load a live report is also a problem:

^^ By the way I thought about something if I'm using the Smart CPM and of course postback so they can see my conversions . Let's say I have a campaign with 500% ROI they won't raise their "Smart CPM" because they have more margin to do that ?
I wouldn't worry too much about that. If they did that sorry of thing it would be a huge potential loss of trust for short term gain.