In the client environments where we've deployed it, we've seen upwards of 40% savings for a manufacturing firm and 31% for a financial services firm. it's a little bit RAM hungry, but a file server with 8GB of RAM is sufficient. Because it uses that extra RAM, IOPS haven't been noticably impacted. I also set it to wait for files to have not been modified for 2 weeks prior to deduping them.
There has been one bizarre interaction between Vipre antivirus and 2012 deduplication at a couple of our clients. It changes a couple of registry keys that prevents Windows XP or OS X clients from accessing the deduplicated data. In those cases, I wound up taking out deduplication. Otherwise, it's worked great.