I use easysignmobile.com I find this the easiest to use on the iPad. I previously used Sign N Send, but easy sign is more streamlined. It will cost you $14.99 for unlimited documents. I recommend you just pay the one time fee instead of per doc if you use it a lot.
We have been using Docu sign and it is a great document. The main issue we have run into is that FHA does not allow electronic signatures at this time and that accounts for many of our deals. For traditional deals docusign is the way to go.