MongoDB 3.0 introduces compression with the WiredTiger storage engine, which supports multiple compression libraries, including Snappy and zlib, offering varying levels of compression rates and resource utilization. Compression is effective for reducing storage needs by decreasing data size, improving disk I/O throughput, and fitting more data in RAM. The benefits of compression include improved performance and reduced storage costs. To test compression with your data in MongoDB 3.0, you can create a new secondary replica set member or restore your data into a standalone instance to compare the compressed sizes of different options. Compression rates vary across databases, such as Enron, Flights, MongoDB Config Database, TPC-H, and Twitter, depending on data characteristics and cardinality. Capped collections are implemented differently in MMAP storage engines compared to WiredTiger, which may affect comparisons between storage engines.