I have worked with Diamond and I have to tell you that they demand a lot. They want to make sure that they have continuity before they will represent your work. On that basis you have to be able to prove to them that you have either the series printed, That you have the material for the series in place and that you will not let them down in any regard.
Because they are the ONLY major distributor of comics they take a hefty amount of commission. Be aware of this if you are getting your budgets together to start printing.
Finally, there are no guarantees of sales. Diamond will show your comics to comic shop owners via an order magazine. The comic shop owners have to want to order your comic. They have very limited shelf space and NEED everything they stock to sell.
It's a tough call. I would never want to put anyone off but don't imagine self publishing and going through Diamond is a sure fire way to success. It isn't. In the same breath, if you have a good story, great artwork and the money to waste if nobody buys your comic then go for it. It's a good feeling to see your comic on the shelf in a good comic store.