I have had some experience with Mongodb and using it for logging. Once it is setup (which is trival) and you have gotten the proper drivers (i used a c# driver with powershell), it is easy to post json documents to the database. It was not quite as easy to get reports out, but the garbage in was no great effort. Run it as a service, get the data from powershell as a scheduled task and pump it in.
There is a really good article in Dr. Dobbs about C# and Mongodb. Actually it is in three different articles: Getting Started with MongoDB, MongoDB with C#, and MongoDB with C# Deep Dive. The third article is really excellent, but it fairly application oriented. The first article is install, and the second writes about MongoDBVue and the C#/Net framework (and a brief comment on using LINQ). It has information that is pretty hard to get normally. Recommended.