I am using OpenX right now on my main project. I just downloaded Oasis to give it a try and I had not heard of Ad Sapient until this post.
I plan on trying all three. The thing I found with OpenX is you better be technical. I am still figuring how to do a easy direct selection and get code that is traffic exchange friendly to generate. The random number thing is annoying too. It seems the system should be able to generate a random number instead of having to insert it yourself. I am using it for ad tracking now and am thinking of exploring the Marketplace functionality. I am an avid TE'er (Traffic Exchanger) so I generate thousands of hits a week to my site and want to start offering some real estate.
I will install Oasis on my local Apache server and see how it looks. I am also going to check out AdSapient. I will report back here on my findings. I am by no means an expert but I will give you my opinion from a real person point of view.