Matrox DualHead2Go and TripleHead2Go also work for Macs, at a fraction of the cost, although on the Macbook (not Pro) you are limited to 2 external monitors each at 1024x768, hooked up to the mini DVI out. The Pro can manage 3 at 800x600 in addition to the LCD, or 3 at 1280x1024 if you disable the built in LCD.
Looking at the Asus device, the review is misleading, as it's only comparing to integrated graphics that is appallingly slow (they're only designed for moving windows around and playing video files). Compared to the X1600 in the MBP the Asus would be a lot slower, as it's limited by the 1x PCI-E channel available to the EC34 slot. Fine if you want to play the latest games at the lowest settings, but not particularly future proof (Might be able to give you a lot of screen real estate though). The price may be pretty high, as essentially the thing is a graphics card and external sound card in one. Also I doubt Asus are interested in making drivers for OSX available (searching asus.com.tw for 'osx' or 'mac' comes up with 0 results).
I confused bits for bytes in my previous post, so here's the correct bandwidth for PCI and PCI-E (MB = megabytes, Mb = megabits):
32bit PCI at 33Mhz = 133MB/s input or output
(Standard short PCI slot in desktop PCs. This is bandwidth is shared between all the slots, and input and output. Servers may have more than one bus to provide more bandwidth, and use PCI-X 64bit slots at up to 533MHz, for a maximum of 4.3GB/s)
PCI-E 1x = simultaneous 256MB/s input and 256MB/s output
PCI-E 16x = simultaneous 4GB/s input and 4GB/s output
(The ExpressCard slot has electrical connections for a PCI-E 1x lane and a USB 2.0 connection. In a typical modern desktop PC motherboard there are 20x PCI-E lanes divided between the slots, so the bandwidth isn't shared like with PCI, and 1 x16 card and 4 x1 cards can run at full speed)
