The microSD card that came with my BeagleBone was a Kingston 4GB, and it has stood up well to frequent re-imaging and on-device development. When I decided to get a second card, I did a little research and chose the SanDisk 4GB microSDHC (model SDSDQY-004G-A11A). I also picked up a Team 16G USB flash drive (model TG016GF108N3), which I use for build directories.
Here are the results from running Bonnie++ on the BeagleBone:
While the huge performance increase of the usb drive dominates the graph, system performance during normal usage (loading applications and and saving files) feels dramatically better with the new microSD.