I've done some testing myself and I use two tools primarily, firstly, the windows 7 copy dialogue and networking dialogue give me a rough idea of how fast transfers happen under realworld conditions. I have a few standardised loads I test it with (big video file, and a 12 gb folder of music).
Lan Speed Test is a slightly more systematic way to test - I use the free version to test transfer a 9gb transfer size (Smaller transfers seem to have lower throughput - I'd test a few sizes to see how it works
It has a nice, simple UI and gives you all the information in a single window. You can install this on multiple clients, and use any share that is accessible to the system as a destination to test on, I typically use windows file shares, but in theory, you could do this to say, a iSCSI share. The paid version adds a server option, so you can choose not to use your hard drive or shared folder as the data dump.
You install it on the systems you want to test from - I just have it on my laptop and move around different network segments, but you can install it on as much, or as few systems as needed.